From c1b5801bb5af0055e5f3d263beaa07026103e212 Mon Sep 17 00:00:00 2001 From: Alexander Motin Date: Fri, 14 Jun 2019 17:07:34 -0400 Subject: [PATCH] Minimize aggsum_compare(&arc_size, arc_c) calls. For busy ARC situation when arc_size close to arc_c is desired. But then it is quite likely that aggsum_compare(&arc_size, arc_c) will need to flush per-CPU buckets to find exact comparison result. Doing that often in a hot path penalizes whole idea of aggsum usage there, since it replaces few simple atomic additions with dozens of lock acquisitions. Replacing aggsum_compare() with aggsum_upper_bound() in code increasing arc_p when ARC is growing (arc_size < arc_c) according to PMC profiles allows to save ~5% of CPU time in aggsum code during sequential write to 12 ZVOLs with 16KB block size on large dual-socket system. I suppose there some minor arc_p behavior change due to lower precision of the new code, but I don't think it is a big deal, since it should affect only very small window in time (aggsum buckets are flushed every second) and in ARC size (buckets are limited to 10 average ARC blocks per CPU). Reviewed-by: Chris Dunlop Reviewed-by: Richard Elling Reviewed-by: George Melikov Reviewed-by: Allan Jude Reviewed-by: Brian Behlendorf Signed-off-by: Alexander Motin Closes #8901 --- module/zfs/arc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/module/zfs/arc.c b/module/zfs/arc.c index 28a6c93c5a..bd2aafc2c2 100644 --- a/module/zfs/arc.c +++ b/module/zfs/arc.c @@ -5613,7 +5613,7 @@ arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag) * If we are growing the cache, and we are adding anonymous * data, and we have outgrown arc_p, update arc_p */ - if (aggsum_compare(&arc_size, arc_c) < 0 && + if (aggsum_upper_bound(&arc_size) < arc_c && hdr->b_l1hdr.b_state == arc_anon && (zfs_refcount_count(&arc_anon->arcs_size) + zfs_refcount_count(&arc_mru->arcs_size) > arc_p))