Date: Wed, 10 Jan 2024 06:20:54 +0000 From: bugzilla-noreply@freebsd.org To: fs@FreeBSD.org Subject: [Bug 275594] High CPU usage by arc_prune; analysis and fix Message-ID: <bug-275594-3630-WfWZyq5Z6Z@https.bugs.freebsd.org/bugzilla/> In-Reply-To: <bug-275594-3630@https.bugs.freebsd.org/bugzilla/> References: <bug-275594-3630@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D275594 --- Comment #28 from Seigo Tanimura <seigo.tanimura@gmail.com> --- (In reply to Seigo Tanimura from comment #22) There was an implementation bug in the test of comment #22. The updated results with the bugfix attached. * Sources on GitHub: - Repo - https://github.com/altimeter-130ft/freebsd-freebsd-src - Branches - Fix only - release/14.0.0/release-14_0_0-p2-topic-openzfs-arc_prune-regulation-f= ix - Fix and counters - release/14.0.0/release-14_0_0-p2-topic-openzfs-arc_prune-regulation-counters - The branches have been renamed to reflect the fix nature. - All of the commits in each branch have been squashed into one. - The commit logs have been revised into the style alike the release note= s. - Patches - Archive: openzfs-arc_prune-regulation-20240110.tar.xz - Each patch is named after its branch. * Bugfix - zfs_prune_task() passed the number of the dnodes to vnlru_free_vfsops() a= nd hence vnlru_free_impl(). - The ARC pruning may overcommit because there are genrally more dnodes t= han znodes. - Fix: Convert the number of the dnodes into that of the znodes before passing it to vnlru_free_vfsops(). - This bug is also found in the original source as well. * Test results Result Chart Archive (1 / 2): (poudriere-bulk-2024-01-09_10h19m59s.7z) - zfs-znodes-and-dnodes.png - The counts of the ZFS znodes and dnodes. - zfs-arc-pruning-regulation.png - The counts of the ARC prune triggers by ZFS and the skips by the fix. - zfs-dnodes-and-freeing-activity.png - The freeing activity of the ZFS znodes and dnodes. - vnode-free-calls.png - The calls to the ZFS vnode freeing functions. Result Chart Archive (2 / 2): (poudriere-bulk-2024-01-09_10h19m59s-zfs-arc.= 7z) - zfs-arc/zfs-arc-meta.png - The balancing of the ZFS ARC metadata and data. - zfs-arc/zfs-arc-(A)-(B)-(C).png - The ZFS ARC stats. (A): MRU (mru) or MFU. (mfu) (B): Metadata (metadata) or data (data); the "ghost-" prefix denotes the evicted cache. (C): Size (size) or hits (hits); the hits count the hit sizes, not the = hit counts. Findings and Analysis: - The giveups of vnlru_free_impl() dropped to virtually zero. - No CPU time was wasted for the effortless ARC pruning. - The logical bug has been fixed. - Some retries are inevitable in order to walk through the vnode list completely. - The actual ARC pruning execution is about 10K less than the requests. - Actual ARC pruning: < 100 / 10 mins. - Max ARC pruning requests: Order of 1M / 10 mins. - Regulation is essential to avoid the uncontrolled behaviour. - Except for the transient time at the start and end of the build, the prun= able znodes stayed under 10% of the total znodes. - 10%: The default value of zfs_arc_dnode_reduce_percent. - The ARC pruning exhibited its best performance under the configured limitation. --=20 You are receiving this mail because: You are the assignee for the bug.=
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