Illumos 5497 - lock contention on arcs_mtx

Reviewed by: George Wilson <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Richard Elling <[email protected]>
Approved by: Dan McDonald <[email protected]>

Porting notes and other significant code changes:

The illumos 5368 patch (ARC should cache more metadata), which
was never picked up by ZoL, is mostly reverted by this patch.

Since ZoL relies on the kernel asynchronously calling the shrinker to
actually reap memory, the shrinker wakes up arc_reclaim_waiters_cv every
time it runs.

The arc_adapt_thread() function no longer calls arc_do_user_evicts()
since the newly-added arc_user_evicts_thread() calls it periodically.

Notable conflicting ZoL commits which conflicted with this patch or
whose effects are either duplicated or un-done by this patch:

    302f753 - Integrate ARC more tightly with Linux
    39e055c - Adjust arc_p based on "bytes" in arc_shrink
    f521ce1 - Allow "arc_p" to drop to zero or grow to "arc_c"
    77765b5 - Remove "arc_meta_used" from arc_adjust calculation
    94520ca - Prune metadata from ghost lists in arc_adjust_meta

Trace support for multilist_insert() and multilist_remove() has been
added and produces the following output:

    fio-12498 [077] .... 112936.448324: zfs_multilist__insert: ml { offset 240 numsublists 80 sublistidx 63 }
    fio-12498 [077] .... 112936.448347: zfs_multilist__remove: ml { offset 240 numsublists 80 sublistidx 29 }

The following arcstats have been removed:

    recycle_miss - Used by arcstat.py and arc_summary.py, both of which
    have been updated appropriately.

    l2_writes_hdr_miss

The following arcstats have been added:

    evict_not_enough - Number of times arc_evict_state() was unable to
    evict enough buffers to reach its target amount.

    evict_l2_skip - Number of times arc_evict_hdr() skipped eviction
    because it was being written to the l2arc.

    l2_writes_lock_retry - Replaces l2_writes_hdr_miss.  Number of times
    l2arc_write_done() failed to acquire hash_lock (and re-tries).

    arc_meta_min - Shows the value of the zfs_arc_meta_min module
    parameter (see below).

The "index" column of the "dbuf" kstat has been removed since it doesn't
have a direct analog in the new multilist scheme.  Additional multilist-
related stats could be added in the future but would likely require
extensions to the mulilist API.

The following module parameters have been added:

    zfs_arc_evict_batch_limit - Number of ARC headers to free per sub-list
    before moving on to the next sub-list.

    zfs_arc_meta_min - Enforce a floor on the amount of metadata in
    the ARC.

    zfs_arc_num_sublists_per_state - Number of multilist sub-lists per
    ARC state.

    zfs_arc_overflow_shift - Controls amount by which the ARC must exceed
    the target size to be considered "overflowing".

Ported-by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]

1 files changed, 60 insertions, 0 deletions

diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index 359e9f72f..250adc9ef 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -350,6 +350,19 @@ Default value: \fB8192\fR.
 .sp
 .ne 2
 .na
+\fBzfs_arc_evict_batch_limit\fR (int)
+.ad
+.RS 12n
+Number ARC headers to evict per sub-list before proceding to another sub-list.
+This batch-style operation prevents entire sub-lists from being evicted at once
+but comes at a cost of additional unlocking and locking.
+.sp
+Default value: \fB10\fR.
+.RE
+
+.sp
+.ne 2
+.na
 \fBzfs_arc_grow_retry\fR (int)
 .ad
 .RS 12n
@@ -398,6 +411,19 @@ Default value: \fB0\fR.
 .sp
 .ne 2
 .na
+\fBzfs_arc_meta_min\fR (ulong)
+.ad
+.RS 12n
+The minimum allowed size in bytes that meta data buffers may consume in
+the ARC.  This value defaults to 0 which disables a floor on the amount
+of the ARC devoted meta data.
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
 \fBzfs_arc_meta_prune\fR (int)
 .ad
 .RS 12n
@@ -450,6 +476,40 @@ Default value: \fB100\fR.
 .sp
 .ne 2
 .na
+\fBzfs_arc_num_sublists_per_state\fR (int)
+.ad
+.RS 12n
+To allow more fine-grained locking, each ARC state contains a series
+of lists for both data and meta data objects.  Locking is performed at
+the level of these "sub-lists".  This parameters controls the number of
+sub-lists per ARC state.
+.sp
+Default value: 1 or the number of on-online CPUs, whichever is greater
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_arc_overflow_shift\fR (int)
+.ad
+.RS 12n
+The ARC size is considered to be overflowing if it exceeds the current
+ARC target size (arc_c) by a threshold determined by this parameter.
+The threshold is calculated as a fraction of arc_c using the formula
+"arc_c >> \fBzfs_arc_overflow_shift\fR".
+
+The default value of 8 causes the ARC to be considered to be overflowing
+if it exceeds the target size by 1/256th (0.3%) of the target size.
+
+When the ARC is overflowing, new buffer allocations are stalled until
+the reclaim thread catches up and the overflow condition no longer exists.
+.sp
+Default value: \fB8\fR.
+.RE
+
+.sp
+.ne 2
+.na
 \fBzfs_arc_p_aggressive_disable\fR (int)
 .ad
 .RS 12n