transport: Reduce pointer usage in Stream structs

[arjan-bal]

The pprof profiles for unary RPC benchmarks indicate significant time spent in runtime.mallocgc and runtime.gcBgMarkWorker. This indicates gRPC is spending significant CPU cycles allocating or garbage collecting.

This change reduces the number of pointer fields in the structs that represent client and server stream. This will reduce number of memory allocations (faster) and also reduce pressure on garbage collector (faster garbage collections) since the GC doesn't need to scan non-pointer fields. For structs which were stored as pointers to ensure values are not copied, a noCopy struct is embedded that will cause go vet to fail if copies are performed. Non-pointer fields are also moved to the end of the struct to improve allocation speed.

Results

There are improvements in QPS, latency and allocs/op for unary RPCs.

# test command
go run benchmark/benchmain/main.go -benchtime=60s -workloads=unary \
   -compression=off -maxConcurrentCalls=500 -trace=off \
   -reqSizeBytes=100 -respSizeBytes=100 -networkMode=Local -resultFile="${RUN_NAME}"   -recvBufferPool=simple

go run benchmark/benchresult/main.go unary-before unary-after       
               Title       Before        After Percentage
            TotalOps      7690250      7991877     3.92%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op     10218.14     10084.00    -1.31%
           Allocs/op       164.85       151.85    -7.89%
             ReqT/op 102536666.67 106558360.00     3.92%
            RespT/op 102536666.67 106558360.00     3.92%
            50th-Lat    3.57283ms   3.435143ms    -3.85%
            90th-Lat   5.152403ms   4.979906ms    -3.35%
            99th-Lat   5.985282ms   5.827893ms    -2.63%
             Avg-Lat    3.89872ms   3.750449ms    -3.80%
           GoVersion     go1.24.4     go1.24.4
         GrpcVersion   1.77.0-dev   1.77.0-dev

Resources

• go/go/performance?polyglot=open-source#application-spends-too-much-on-gc-or-allocations
• go/go/performance?polyglot=open-source#memory-optimizations

RELEASE NOTES:

• transport: Reduce pointer usage to lower garbage collection pressure and improve unary RPC performance.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.15%. Comparing base (d0ebcdf) to head (0a52b64).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/transport/transport.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8624      +/-   ##
==========================================
+ Coverage   82.12%   82.15%   +0.03%     
==========================================
  Files         415      415              
  Lines       40701    40711      +10     
==========================================
+ Hits        33425    33447      +22     
+ Misses       5895     5884      -11     
+ Partials     1381     1380       -1     

Files with missing lines	Coverage Δ
internal/transport/client_stream.go	100.00% <ø> (ø)
internal/transport/flowcontrol.go	96.46% <100.00%> (-0.04%)	⬇️
internal/transport/handler_server.go	90.84% <100.00%> (+0.03%)	⬆️
internal/transport/http2_client.go	92.15% <100.00%> (-0.08%)	⬇️
internal/transport/http2_server.go	90.59% <100.00%> (-0.28%)	⬇️
internal/transport/server_stream.go	100.00% <ø> (+4.68%)	⬆️
internal/transport/transport.go	90.80% <50.00%> (+6.06%)	⬆️

... and 19 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[easwars]

This syntax does feel a little weird to me. Did you try some of these options to see if they read better (and don't perform worse)?

• directly set fields of wq in here instead of setting wq to a completely new instance of writeQuota
• Can this initWriteQuota be a method on Stream?
• Can we make the zero value of writeQouta something that can actually work?

This comment applies to other types as well like recvBuffer.

Thanks.