Backtrack execution for missing digests to make eager_fetch=false more resilient
#15850
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This was referenced Jun 16, 2022
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tdyas
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| /// | ||
| pub fn maybe_start_backtracking(&self, node: &ExecuteProcess) -> usize { | ||
| let mut backtrack_attempts = self.backtrack_attempts.lock(); | ||
| let entry: Option<&mut usize> = backtrack_attempts.get_mut(node); |
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Rust could not infer type ascription?
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This is defensive, and partially superstition: if the field is dereferenced too early, the increment can be lost (because you are incrementing a local variable rather than the value behind the pointer). The compiler still won't catch that IIRC.
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[ci skip-build-wheels]
# Building wheels and fs_util will be skipped. Delete if not intended. [ci skip-build-wheels]
…an integration test. [ci skip-build-wheels]
[ci skip-build-wheels]
[ci skip-build-wheels]
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Rather than #15856, I've added one more commit here that merges the |
# Building wheels and fs_util will be skipped. Delete if not intended. [ci skip-build-wheels]
tdyas
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Jun 17, 2022
# Building wheels and fs_util will be skipped. Delete if not intended. [ci skip-build-wheels]
illicitonion
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Jun 17, 2022
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As described in #11331, in order to avoid having to deal with missing remote content later in the pipeline,
--remote-cache-eager-fetchcurrently defaults to true. This means that before calling a cache hit a hit, we fully download the output of the cache entry.In warm-cache situations, this can mean downloading a lot more than is strictly necessary. In theory, you could imagine
eager_fetch=Falsedownloading only stdio and no file content at all for a 100% cache hit rate run of tests. In practice, high hitrate runs see about 80% fewer bytes downloaded, and 50% fewer RPCs than witheager_fetch=True.To begin moving toward disabling
eager_fetchby default (and eventually, ideally, removing the flag entirely), this change begins "backtracking" when missing digests are encountered. Backtracking is implemented by "catching"MissingDigesterrors (introduced in #15761), and invalidating their sourceNodein the graph. When aNodethat produced a missing digest re-runs, it does so using progressively fewer caches (as introduced in #15854), in order to cache bust both local and remote partial cache entries.eager_fetch=Falsewas already experimental, in that anyMissingDigesterror encountered later in the run would kill the entire run. Backtracking makeseager_fetch=Falseless experimental, in that we are now very likely to recover from aMissingDigesterror. But it is still the case witheager_fetch=Falsethat persistent remote infrastructure errors (those that last longer than our retry budget or timeout) could kill a run. Given that, we will likely want to gain more experience and further tune timeouts and retries before changing the default.Fixes #11331.