Speed up cycle detector reaping some actors #3649

SeanTAllen · 2020-09-10T17:52:41Z

This commit speeds up reaping of some actors by the cycle-detector.
When an actor is done running, we do a check similar to the check
we do for when --ponynoblock is on:

Check to see if the actor's message queue is empty
Check to see if it has a reference count of 0

If both are true, then the actor can be deleted. When --ponynoblock
is used, this is done immediately. It can't be done immediately when
the cycle detector is in use because, it might have references to
the actor in question.

In order to safely cleanup and reap actors, the cycle detector must
be the one to do the reaping. This would eventually be done without
this change. However, it is possible to write programs where the
speed at which the cycle detectors protocol will operate is unable
to keep up with the speed at which new "one shot" actors are created.

Here's an example of such a program:

actor Main
  new create(e: Env) =>
    Spawner.run()

actor Spawner
  var _living_children: U64 = 0

  new create() =>
    None

  be run() =>
    _living_children = _living_children + 1
    Spawnee(this).run()

  be collect() =>
    _living_children = _living_children - 1
    run()

actor Spawnee
  let _parent: Spawner

  new create(parent: Spawner) =>
    _parent = parent

  be run() =>
    _parent.collect()

Without this patch, that program will have large amount of memory growth
and eventually, run out of memory. With the patch, Spawnee actors will be
reaped much more quickly and stable memory growth can be achieved.

This is not a complete solution to the problem as issue #1007 is still a
problem due to reasons I outlined at
#1007 (comment).

It should also be noted that in the process of developing this patch,
Dipin Hora and I discovered a use after free race condition in the
runtime mpmcq implementation. That race condition can and will result
in segfaults when running the above example program. There's no issue
opened yet for that problem, but one will be coming.

While my name is on the commit, credit should be divided up between myself
and Dipin. I came up with the basic idea of how to reap some "one shot"
actors earlier and Dipin reimplemented by my idea to work much better with
the existing cycle-detector than my approach which was overly complicated and
had a number of areas that were likely sources of subtle bugs.

ponylang-main · 2020-09-10T19:32:02Z

Hi @SeanTAllen,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3649.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

SeanTAllen · 2020-09-10T20:08:44Z

Windows errors appear to be related to stdlib tests not being run sequentially for which I've opened a PR:

#3651

This commit speeds up reaping of some actors by the cycle-detector. When an actor is done running, we do a check similar to the check we do for when --ponynoblock is on: - Check to see if the actor's message queue is empty - Check to see if it has a reference count of 0 If both are true, then the actor can be deleted. When --ponynoblock is used, this is done immediately. It can't be done immediately when the cycle detector is in use because, it might have references to the actor in question. In order to safely cleanup and reap actors, the cycle detector must be the one to do the reaping. This would eventually be done without this change. However, it is possible to write programs where the speed at which the cycle detectors protocol will operate is unable to keep up with the speed at which new "one shot" actors are created. Here's an example of such a program: ```pony actor Main new create(e: Env) => Spawner.run() actor Spawner var _living_children: U64 = 0 new create() => None be run() => _living_children = _living_children + 1 Spawnee(this).run() be collect() => _living_children = _living_children - 1 run() actor Spawnee let _parent: Spawner new create(parent: Spawner) => _parent = parent be run() => _parent.collect() ``` Without this patch, that program will have large amount of memory growth and eventually, run out of memory. With the patch, Spawnee actors will be reaped much more quickly and stable memory growth can be achieved. This is not a complete solution to the problem as issue #1007 is still a problem due to reasons I outlined at #1007 (comment). It should also be noted that in the process of developing this patch, Dipin Hora and I discovered a use after free race condition in the runtime mpmcq implementation. That race condition can and will result in segfaults when running the above example program. There's no issue opened yet for that problem, but one will be coming. While my name is on the commit, credit should be divided up between myself and Dipin. I came up with the basic idea of how to reap some "one shot" actors earlier and Dipin reimplemented by my idea to work much better with the existing cycle-detector than my approach which was overly complicated and had a number of areas that were likely sources of subtle bugs.

SeanTAllen · 2020-09-11T13:05:27Z

Windows errors appear to be related to stdlib tests not being run sequentially for which I've opened a PR:

#3651

I fixed this in another commit and rebased over it.

@printf

We have a few example programs that create a large number of short-lived actors that can exhibit runaway memory growth. The changes in this commit greatly reduce the memory growth or potentially reduce it to be stable. The primary change here is to use some static analysis at compilation time to determine if an actor is "an orphan". That is, upon creation, has no actors (including it's creator) with references to it. In order to support the concept of an "orphan actor", additional changes had to be made to the cycle detector protocol and how other actors interact with it. There are two primary things we can do with an orphan actor to improve the rate of garbage collection when the cycle detector is running. When the cycle detector is running actors are collected by the cycle detector when they have no other actors referring to them and can't receive any new messages or are in a cycle that can't receive additional messages. The examples listed in this PR all had problematic memory usage because, actors were created at a rate that overwhelmed the cycle detector and caused large amount of messages to it to back up and sit in its queue waiting to be processed. This commit addresses the "too many messages" problem by making the following actor GC rule changes for orphan actors. Note, all these rules are when the cycle detector is running. * when an actor finishes being run by a scheduler thread "locally delete" it if: * no other actors hold a reference to the actor * the cycle detector isn't holding a reference to the actor "locally deleting" follows the same message free protocol used when the cycle detector isn't running. * when an actor finishes being run by a scheduler thread, pre-emptively inform the cycle detector that we are blocked if: * no other actors hold a reference to the actor * the cycle detector is holding a reference to the actor by pre-emptively informing the cycle detector that we are blocked, the cycle detector can delete the actor more quickly. This "pre-emptively" notify change was introduced in #3649. In order to support these changes, additional cycle detector protocol changes were made. Previously, every actor upon creation informed the cycle detector of its existence. If we want to allow for local deleting, then this won't work. Every actor was known by the cycle detector. All concept of "actor created" messages have been removed by this change. Instead, actors have a flag FLAG_CD_CONTACTED that is set if the actor has ever sent a message to the cycle detector. Once the flag is set, we know that locally deleting the actor is unsafe and we can fall back on the slower pre-emptively inform strategy. The cycle detector works by periodically sending messages to all actors it knows about and asking them if they are currently blocked as the first step in its "path to actor garbage collection" protocol. As actors no longer inform the cycle detector of their existence on creation, the cycle detector needs a new way to discover actors. The first time an actor becomes logically blocked, it sets itself as blocked and notifies the cycle detector of its existence and that it is blocked. This is done by: * setting the actor as blocked * sending an "i'm blocked" message to the cycle detector * setting FLAG_CD_CONTACTED on the actor The overall effect of these changes is: - The rate of garbage collection for short-lived actors is improved. - Fewer cycle detector related messages are generated. It should be noted that if an actor is incorrectly identified by the compiler as being an orphan when it is in fact not, the end result would be a segfault. During development of this feature, it was discovered that the following code from the Promise select test would segfault: ```pony let pb = Promise[String] .> next[None]({(s) => h.complete_action(s) }) ``` The issue was that .> wasn't being correctly handled by the `is_result_needed` logic in `expr.c`. It is possibly that other logic within the function is faulty and if segfaults are see after this commit that didn't exist prior to it, then incorrectly identifying an actor as an orphan might be the culprit. Prior to these change, examples such as the one from issue #1007 (listed later) would be unable to be collected due to an edge-case in the cycle detector and the runtime garbage collection algorithms. Issue #1007 was opened with the following code having explosive memory growth: ```pony primitive N fun apply(): U64 => 2_000_000_000 actor Test new create(n: U64) => if n == 0 then return end Test(n - 1) actor Main new create(env: Env) => Test(N()) ``` Dipin and I previously worked on #3649 which added "pre-emptively informing" as detailed earlier. The example above from #1007 wasn't addressed by #3649 because the key to #3649 was that when done running its behaviors, an actor can see if it has no additional messages AND no references to itself and can then tell the cycle detector to skip parts of the CD protocol and garbage collect sooner. #1007 requires the "local delete" functionality from this commit whereas #3649 only provided "pre-emptive informing". The addition of "local delete" allows for the cycle detector to keep up with many cases of generating large amounts of "orphaned" actors. The from #1007 above wasn't addressed by #3649 because of an implementation detail in the ORCA garbage collection protocol; at the time that an instance of Test is done running its create behavior, it doesn't have a reference count of 0. It has a reference count of 256. Not because there are 256 references but because, when an actor is created puts a "fake value" in the rc value such that an actor isn't gc'd prematurely. The value will be set to the correct value once the actor that created it is first traced and will be subsequently updated correctly per ORCA going forward. However, at the time that an instance of Test is finished running its create, that information isn't available. It would be incorrect to say "if rc is 256, I'm blocked and you can gc me". 256 is a perfectly reasonable value for an rc to be in normal usage. This isn't a problem with the changes in this PR as the compiler detects that each instance of Test will be an orphan and immediately sets its rc to 0. This allows it to be garbage collected as the instance's message queue is empty so long as it's rc remains 0. Any changes in the future to address lingering issues with creating large numbers of orphaned actors should also be tested with the following examples. Each exercises a different type of pattern that could lead to memory growth or incorrect execution. Example 2 features reasonably stable memory usage that I have seen from time-to-time, increase rapidly. It should be noted that such an increase is rather infrequent but suggests there are additional problems in the cycle-detector. I suspect said problem is a periodic burst of additional messages to the cycle-detector from actors that can be garbage collected, but I haven't investigated further. ```pony actor Main new create(e: Env) => Spawner.run() actor Spawner var _living_children: U64 = 0 new create() => None be run() => _living_children = _living_children + 1 Spawnee(this).run() be collect() => _living_children = _living_children - 1 run() actor Spawnee let _parent: Spawner new create(parent: Spawner) => _parent = parent be run() => _parent.collect() ``` Example 3 has stable memory growth and given that it won't result in any messages being sent to the cycle detector as we have determined at compile-time that the Foo actor instances are orphaned. ```pony actor Main var n: U64 = 2_000_000_000 new create(e: Env) => run() be run() => while(n > 0 ) do Foo(n) n = n - 1 if ((n % 1_000) == 0) then run() break end end actor Foo new create(n: U64) => if ((n % 1_000_000) == 0) then @printf[I32]("%ld\n".cstring(), n) end None ``` Example 4 has the same characteristics as example 3 with the code as of this commit. However, it did exhibit different behavior prior to this commit being fully completed and appears to be a good test candidate for any future changes. ```pony actor Main var n: U64 = 2_000_000_000 new create(e: Env) => run() be run() => while(n > 0 ) do Foo(n) n = n - 1 if ((n % 1_000_000) == 0) then @printf[I32]("%ld\n".cstring(), n) end if ((n % 1_000) == 0) then run() break end end actor Foo new create(n: U64) => None ``` Finally, for anyone who works on improving this in the future, here's an additional test program beyond ones that already exist elsewhere for testing pony programs. This program will create a large number of actors that are orphaned but then send themselves to another actor. This should increase their rc count and keep them from being garbage collected. If the program segfaults, then something has gone wrong and the logic related to orphan actors has been broken. The example currently passes as of this commit. ```pony use "collections" actor Holder let _holding: SetIs[ManyOfMe] = _holding.create() new create() => None be hold(a: ManyOfMe) => _holding.set(a) be ping() => var c: U64 = 1_000 for h in _holding.values() do if c > 0 then h.ping() c = c - 1 end end actor ManyOfMe new create(h: Holder) => h.hold(this) be ping() => None actor Main var n: U64 = 500_000 let holder: Holder new create(env: Env) => holder = Holder run() be run() => while(n > 0 ) do ManyOfMe(holder) n = n - 1 if ((n % 1_000) == 0) then @printf[I32]("%ld\n".cstring(), n) run() holder.ping() break end end ``` The code and ideas in this commit, while attributed to me is the result of a group effort featuring ideas and/or code from myself, Joe Eli McIlvain, Dipin Hora, and Sylvan Clebsch. Closes #1007

@printf

We have a few example programs that create a large number of short-lived actors that can exhibit runaway memory growth. The changes in this commit greatly reduce the memory growth or potentially reduce it to be stable. The primary change here is to use some static analysis at compilation time to determine if an actor is "an orphan". That is, upon creation, has no actors (including it's creator) with references to it. In order to support the concept of an "orphan actor", additional changes had to be made to the cycle detector protocol and how other actors interact with it. There are two primary things we can do with an orphan actor to improve the rate of garbage collection when the cycle detector is running. When the cycle detector is running actors are collected by the cycle detector when they have no other actors referring to them and can't receive any new messages or are in a cycle that can't receive additional messages. The examples listed in this PR all had problematic memory usage because, actors were created at a rate that overwhelmed the cycle detector and caused large amount of messages to it to back up and sit in its queue waiting to be processed. This commit addresses the "too many messages" problem by making the following actor GC rule changes for orphan actors. Note, all these rules are when the cycle detector is running. * when an actor finishes being run by a scheduler thread "locally delete" it if: * no other actors hold a reference to the actor * the cycle detector isn't holding a reference to the actor "locally deleting" follows the same message free protocol used when the cycle detector isn't running. * when an actor finishes being run by a scheduler thread, pre-emptively inform the cycle detector that we are blocked if: * no other actors hold a reference to the actor * the cycle detector is holding a reference to the actor by pre-emptively informing the cycle detector that we are blocked, the cycle detector can delete the actor more quickly. This "pre-emptively" notify change was introduced in #3649. In order to support these changes, additional cycle detector protocol changes were made. Previously, every actor upon creation informed the cycle detector of its existence. If we want to allow for local deleting, then this won't work. Every actor was known by the cycle detector. All concept of "actor created" messages have been removed by this change. Instead, actors have a flag FLAG_CD_CONTACTED that is set if the actor has ever sent a message to the cycle detector. Once the flag is set, we know that locally deleting the actor is unsafe and we can fall back on the slower pre-emptively inform strategy. The cycle detector works by periodically sending messages to all actors it knows about and asking them if they are currently blocked as the first step in its "path to actor garbage collection" protocol. As actors no longer inform the cycle detector of their existence on creation, the cycle detector needs a new way to discover actors. The first time an actor becomes logically blocked, it sets itself as blocked and notifies the cycle detector of its existence and that it is blocked. This is done by: * setting the actor as blocked * sending an "i'm blocked" message to the cycle detector * setting FLAG_CD_CONTACTED on the actor The overall effect of these changes is: - The rate of garbage collection for short-lived actors is improved. - Fewer cycle detector related messages are generated. It should be noted that if an actor is incorrectly identified by the compiler as being an orphan when it is in fact not, the end result would be a segfault. During development of this feature, it was discovered that the following code from the Promise select test would segfault: ```pony let pb = Promise[String] .> next[None]({(s) => h.complete_action(s) }) ``` The issue was that .> wasn't being correctly handled by the `is_result_needed` logic in `expr.c`. It is possibly that other logic within the function is faulty and if segfaults are see after this commit that didn't exist prior to it, then incorrectly identifying an actor as an orphan might be the culprit. Prior to these change, examples such as the one from issue #1007 (listed later) would be unable to be collected due to an edge-case in the cycle detector and the runtime garbage collection algorithms. Issue #1007 was opened with the following code having explosive memory growth: ```pony primitive N fun apply(): U64 => 2_000_000_000 actor Test new create(n: U64) => if n == 0 then return end Test(n - 1) actor Main new create(env: Env) => Test(N()) ``` Dipin and I previously worked on #3649 which added "pre-emptively informing" as detailed earlier. The example above from #1007 wasn't addressed by #3649 because the key to #3649 was that when done running its behaviors, an actor can see if it has no additional messages AND no references to itself and can then tell the cycle detector to skip parts of the CD protocol and garbage collect sooner. #1007 requires the "local delete" functionality from this commit whereas #3649 only provided "pre-emptive informing". The addition of "local delete" allows for the cycle detector to keep up with many cases of generating large amounts of "orphaned" actors. The from #1007 above wasn't addressed by #3649 because of an implementation detail in the ORCA garbage collection protocol; at the time that an instance of Test is done running its create behavior, it doesn't have a reference count of 0. It has a reference count of 256. Not because there are 256 references but because, when an actor is created puts a "fake value" in the rc value such that an actor isn't gc'd prematurely. The value will be set to the correct value once the actor that created it is first traced and will be subsequently updated correctly per ORCA going forward. However, at the time that an instance of Test is finished running its create, that information isn't available. It would be incorrect to say "if rc is 256, I'm blocked and you can gc me". 256 is a perfectly reasonable value for an rc to be in normal usage. This isn't a problem with the changes in this PR as the compiler detects that each instance of Test will be an orphan and immediately sets its rc to 0. This allows it to be garbage collected as the instance's message queue is empty so long as it's rc remains 0. Any changes in the future to address lingering issues with creating large numbers of orphaned actors should also be tested with the following examples. Each exercises a different type of pattern that could lead to memory growth or incorrect execution. Example 2 features reasonably stable memory usage that I have seen from time-to-time, increase rapidly. It should be noted that such an increase is rather infrequent but suggests there are additional problems in the cycle-detector. I suspect said problem is a periodic burst of additional messages to the cycle-detector from actors that can be garbage collected, but I haven't investigated further. ```pony actor Main new create(e: Env) => Spawner.run() actor Spawner var _living_children: U64 = 0 new create() => None be run() => _living_children = _living_children + 1 Spawnee(this).run() be collect() => _living_children = _living_children - 1 run() actor Spawnee let _parent: Spawner new create(parent: Spawner) => _parent = parent be run() => _parent.collect() ``` Example 3 has stable memory growth and given that it won't result in any messages being sent to the cycle detector as we have determined at compile-time that the Foo actor instances are orphaned. ```pony actor Main var n: U64 = 2_000_000_000 new create(e: Env) => run() be run() => while(n > 0 ) do Foo(n) n = n - 1 if ((n % 1_000) == 0) then run() break end end actor Foo new create(n: U64) => if ((n % 1_000_000) == 0) then @printf[I32]("%ld\n".cstring(), n) end None ``` Example 4 has the same characteristics as example 3 with the code as of this commit. However, it did exhibit different behavior prior to this commit being fully completed and appears to be a good test candidate for any future changes. ```pony actor Main var n: U64 = 2_000_000_000 new create(e: Env) => run() be run() => while(n > 0 ) do Foo(n) n = n - 1 if ((n % 1_000_000) == 0) then @printf[I32]("%ld\n".cstring(), n) end if ((n % 1_000) == 0) then run() break end end actor Foo new create(n: U64) => None ``` Finally, for anyone who works on improving this in the future, here's an additional test program beyond ones that already exist elsewhere for testing pony programs. This program will create a large number of actors that are orphaned but then send themselves to another actor. This should increase their rc count and keep them from being garbage collected. If the program segfaults, then something has gone wrong and the logic related to orphan actors has been broken. The example currently passes as of this commit. ```pony use "collections" actor Holder let _holding: SetIs[ManyOfMe] = _holding.create() new create() => None be hold(a: ManyOfMe) => _holding.set(a) be ping() => var c: U64 = 1_000 for h in _holding.values() do if c > 0 then h.ping() c = c - 1 end end actor ManyOfMe new create(h: Holder) => h.hold(this) be ping() => None actor Main var n: U64 = 500_000 let holder: Holder new create(env: Env) => holder = Holder run() be run() => while(n > 0 ) do ManyOfMe(holder) n = n - 1 if ((n % 1_000) == 0) then @printf[I32]("%ld\n".cstring(), n) run() holder.ping() break end end ``` The code and ideas in this commit, while attributed to me is the result of a group effort featuring ideas and/or code from myself, Joe Eli McIlvain, Dipin Hora, and Sylvan Clebsch. Closes #1007

@printf

We have a few example programs that create a large number of short-lived actors that can exhibit runaway memory growth. The changes in this commit greatly reduce the memory growth or potentially reduce it to be stable. The primary change here is to use some static analysis at compilation time to determine if an actor is "an orphan". That is, upon creation, has no actors (including it's creator) with references to it. In order to support the concept of an "orphan actor", additional changes had to be made to the cycle detector protocol and how other actors interact with it. There are two primary things we can do with an orphan actor to improve the rate of garbage collection when the cycle detector is running. When the cycle detector is running actors are collected by the cycle detector when they have no other actors referring to them and can't receive any new messages or are in a cycle that can't receive additional messages. The examples listed in this PR all had problematic memory usage because, actors were created at a rate that overwhelmed the cycle detector and caused large amount of messages to it to back up and sit in its queue waiting to be processed. This commit addresses the "too many messages" problem by making the following actor GC rule changes for orphan actors. Note, all these rules are when the cycle detector is running. * when an actor finishes being run by a scheduler thread "locally delete" it if: * no other actors hold a reference to the actor * the cycle detector isn't holding a reference to the actor "locally deleting" follows the same message free protocol used when the cycle detector isn't running. * when an actor finishes being run by a scheduler thread, pre-emptively inform the cycle detector that we are blocked if: * no other actors hold a reference to the actor * the cycle detector is holding a reference to the actor by pre-emptively informing the cycle detector that we are blocked, the cycle detector can delete the actor more quickly. This "pre-emptively" notify change was introduced in #3649. In order to support these changes, additional cycle detector protocol changes were made. Previously, every actor upon creation informed the cycle detector of its existence. If we want to allow for local deleting, then this won't work. Every actor was known by the cycle detector. All concept of "actor created" messages have been removed by this change. Instead, actors have a flag FLAG_CD_CONTACTED that is set if the actor has ever sent a message to the cycle detector. Once the flag is set, we know that locally deleting the actor is unsafe and we can fall back on the slower pre-emptively inform strategy. The cycle detector works by periodically sending messages to all actors it knows about and asking them if they are currently blocked as the first step in its "path to actor garbage collection" protocol. As actors no longer inform the cycle detector of their existence on creation, the cycle detector needs a new way to discover actors. The first time an actor becomes logically blocked, it sets itself as blocked and notifies the cycle detector of its existence and that it is blocked. This is done by: * setting the actor as blocked * sending an "i'm blocked" message to the cycle detector * setting FLAG_CD_CONTACTED on the actor The overall effect of these changes is: - The rate of garbage collection for short-lived actors is improved. - Fewer cycle detector related messages are generated. It should be noted that if an actor is incorrectly identified by the compiler as being an orphan when it is in fact not, the end result would be a segfault. During development of this feature, it was discovered that the following code from the Promise select test would segfault: ```pony let pb = Promise[String] .> next[None]({(s) => h.complete_action(s) }) ``` The issue was that .> wasn't being correctly handled by the `is_result_needed` logic in `expr.c`. It is possibly that other logic within the function is faulty and if segfaults are see after this commit that didn't exist prior to it, then incorrectly identifying an actor as an orphan might be the culprit. Prior to these change, examples such as the one from issue #1007 (listed later) would be unable to be collected due to an edge-case in the cycle detector and the runtime garbage collection algorithms. Issue #1007 was opened with the following code having explosive memory growth: ```pony primitive N fun apply(): U64 => 2_000_000_000 actor Test new create(n: U64) => if n == 0 then return end Test(n - 1) actor Main new create(env: Env) => Test(N()) ``` Dipin and I previously worked on #3649 which added "pre-emptively informing" as detailed earlier. The example above from #1007 wasn't addressed by #3649 because the key to #3649 was that when done running its behaviors, an actor can see if it has no additional messages AND no references to itself and can then tell the cycle detector to skip parts of the CD protocol and garbage collect sooner. #1007 requires the "local delete" functionality from this commit whereas #3649 only provided "pre-emptive informing". The addition of "local delete" allows for the cycle detector to keep up with many cases of generating large amounts of "orphaned" actors. The from #1007 above wasn't addressed by #3649 because of an implementation detail in the ORCA garbage collection protocol; at the time that an instance of Test is done running its create behavior, it doesn't have a reference count of 0. It has a reference count of 256. Not because there are 256 references but because, when an actor is created puts a "fake value" in the rc value such that an actor isn't gc'd prematurely. The value will be set to the correct value once the actor that created it is first traced and will be subsequently updated correctly per ORCA going forward. However, at the time that an instance of Test is finished running its create, that information isn't available. It would be incorrect to say "if rc is 256, I'm blocked and you can gc me". 256 is a perfectly reasonable value for an rc to be in normal usage. This isn't a problem with the changes in this PR as the compiler detects that each instance of Test will be an orphan and immediately sets its rc to 0. This allows it to be garbage collected as the instance's message queue is empty so long as it's rc remains 0. Any changes in the future to address lingering issues with creating large numbers of orphaned actors should also be tested with the following examples. Each exercises a different type of pattern that could lead to memory growth or incorrect execution. Example 2 features reasonably stable memory usage that I have seen from time-to-time, increase rapidly. It should be noted that such an increase is rather infrequent but suggests there are additional problems in the cycle-detector. I suspect said problem is a periodic burst of additional messages to the cycle-detector from actors that can be garbage collected, but I haven't investigated further. ```pony actor Main new create(e: Env) => Spawner.run() actor Spawner var _living_children: U64 = 0 new create() => None be run() => _living_children = _living_children + 1 Spawnee(this).run() be collect() => _living_children = _living_children - 1 run() actor Spawnee let _parent: Spawner new create(parent: Spawner) => _parent = parent be run() => _parent.collect() ``` Example 3 has stable memory growth and given that it won't result in any messages being sent to the cycle detector as we have determined at compile-time that the Foo actor instances are orphaned. ```pony actor Main var n: U64 = 2_000_000_000 new create(e: Env) => run() be run() => while(n > 0 ) do Foo(n) n = n - 1 if ((n % 1_000) == 0) then run() break end end actor Foo new create(n: U64) => if ((n % 1_000_000) == 0) then @printf[I32]("%ld\n".cstring(), n) end None ``` Example 4 has the same characteristics as example 3 with the code as of this commit. However, it did exhibit different behavior prior to this commit being fully completed and appears to be a good test candidate for any future changes. ```pony actor Main var n: U64 = 2_000_000_000 new create(e: Env) => run() be run() => while(n > 0 ) do Foo(n) n = n - 1 if ((n % 1_000_000) == 0) then @printf[I32]("%ld\n".cstring(), n) end if ((n % 1_000) == 0) then run() break end end actor Foo new create(n: U64) => None ``` Finally, for anyone who works on improving this in the future, here's an additional test program beyond ones that already exist elsewhere for testing pony programs. This program will create a large number of actors that are orphaned but then send themselves to another actor. This should increase their rc count and keep them from being garbage collected. If the program segfaults, then something has gone wrong and the logic related to orphan actors has been broken. The example currently passes as of this commit. ```pony use "collections" actor Holder let _holding: SetIs[ManyOfMe] = _holding.create() new create() => None be hold(a: ManyOfMe) => _holding.set(a) be ping() => var c: U64 = 1_000 for h in _holding.values() do if c > 0 then h.ping() c = c - 1 end end actor ManyOfMe new create(h: Holder) => h.hold(this) be ping() => None actor Main var n: U64 = 500_000 let holder: Holder new create(env: Env) => holder = Holder run() be run() => while(n > 0 ) do ManyOfMe(holder) n = n - 1 if ((n % 1_000) == 0) then @printf[I32]("%ld\n".cstring(), n) run() holder.ping() break end end ``` The code and ideas in this commit, while attributed to me is the result of a group effort featuring ideas and/or code from myself, Joe Eli McIlvain, Dipin Hora, and Sylvan Clebsch. Closes #1007

SeanTAllen requested a review from a team September 10, 2020 17:52

SeanTAllen force-pushed the dipin-cd-patch branch from 070c038 to 2e07377 Compare September 10, 2020 17:58

SeanTAllen added the changelog - fixed Automatically add "Fixed" CHANGELOG entry on merge label Sep 10, 2020

SeanTAllen force-pushed the dipin-cd-patch branch from 2e07377 to bfbbdf3 Compare September 10, 2020 19:35

SeanTAllen force-pushed the dipin-cd-patch branch from bfbbdf3 to 610f217 Compare September 10, 2020 20:54

jemc approved these changes Sep 11, 2020

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SeanTAllen merged commit f920092 into master Sep 11, 2020

SeanTAllen deleted the dipin-cd-patch branch September 11, 2020 15:34

github-actions bot pushed a commit that referenced this pull request Sep 11, 2020

Updates release notes for PR #3649

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github-actions bot pushed a commit that referenced this pull request Sep 11, 2020

Update CHANGELOG for PR #3649

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SeanTAllen mentioned this pull request Sep 19, 2020

Improvements to garbage collection of short-lived actors #3653

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Speed up cycle detector reaping some actors #3649

Speed up cycle detector reaping some actors #3649

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SeanTAllen commented Sep 11, 2020

Speed up cycle detector reaping some actors #3649

Speed up cycle detector reaping some actors #3649

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