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https://issues.apache.org/jira/browse/MRESOLVER-346?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17704137#comment-17704137
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Michael Osipov commented on MRESOLVER-346:
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The first statement isn't true. Those eager locks have been available since the
sync context has been introduced. The difference is that previously it was a
noop.
> Too eager locking
> -----------------
>
> Key: MRESOLVER-346
> URL: https://issues.apache.org/jira/browse/MRESOLVER-346
> Project: Maven Resolver
> Issue Type: Bug
> Components: Resolver
> Reporter: Tamas Cservenak
> Priority: Major
> Fix For: 1.9.8
>
>
> The locking that is present in Resolver since 1.8.0 is too eager:
> * there are no shared locks used at all
> * this implies that local repository access is heavily serialized by locking
> * there is no "upgrade" of locking due that above
> * consequence is that "hot" artifacts in bigger multi module build run in
> parallel become bottleneck as all threads will wait for their moment to grab
> exclusive lock.
> * another consequence: our "wait time" (def 30s) becomes problem, as due that
> above, if build grows, the plausible "wait time" (as all lock is exclusive,
> but requester count grows) grows as well. Also, this means we have threads
> there doing nothing, just sitting as they wait for exclusive lock one after
> another.
> We can do it better: there are 4 main areas where locking is used:
> * ArtifactInstaller: it is about to install (write) artifact files to local
> repository, it needs exclusive lock, *no change needed*.
> * ArtifactDeployer: it is about to upload present files to remote, it does
> not modifies anything on local. *Change it's lock to shared*. The exclusive
> lock also meant that if no DeployAtEnd used, other modules during resolution
> had to wait while this module uploaded.
> * ArtifactResolver and MetadataResolver: two very similar beasts, they
> attempt to resolve locally (from local repo) w/o any content modification
> (read only), and if not successful, they will reach remote to download what
> is needed (write). Here we *could do something similar to
> [DCL|https://en.wikipedia.org/wiki/Double-checked_locking] is*: try with
> shared lock first, and if local content is not fulfilling, release shared,
> acquire exclusive and REDO all (as meanwhile someone else may downloaded
> files already).
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