--- /dev/null
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Brian Behlendorf <behlendorf1@llnl.gov>
+Date: Mon, 8 Jan 2018 09:52:36 -0800
+Subject: [PATCH] Fix ARC hit rate
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+When the compressed ARC feature was added in commit d3c2ae1
+the method of reference counting in the ARC was modified. As
+part of this accounting change the arc_buf_add_ref() function
+was removed entirely.
+
+This would have be fine but the arc_buf_add_ref() function
+served a second undocumented purpose of updating the ARC access
+information when taking a hold on a dbuf. Without this logic
+in place a cached dbuf would not migrate its associated
+arc_buf_hdr_t to the MFU list. This would negatively impact
+the ARC hit rate, particularly on systems with a small ARC.
+
+This change reinstates the missing call to arc_access() from
+dbuf_hold() by implementing a new arc_buf_access() function.
+
+Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
+Reviewed-by: Tony Hutter <hutter2@llnl.gov>
+Reviewed-by: Tim Chase <tim@chase2k.com>
+Reviewed by: George Wilson <george.wilson@delphix.com>
+Reviewed-by: George Melikov <mail@gmelikov.ru>
+Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
+Closes #6171
+Closes #6852
+Closes #6989
+Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
+---
+ include/sys/arc.h | 1 +
+ module/zfs/arc.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
+ module/zfs/dbuf.c | 4 +++-
+ 3 files changed, 56 insertions(+), 3 deletions(-)
+
+diff --git a/include/sys/arc.h b/include/sys/arc.h
+index 66f37cf71..1ea4937bd 100644
+--- a/include/sys/arc.h
++++ b/include/sys/arc.h
+@@ -221,6 +221,7 @@ void arc_buf_destroy(arc_buf_t *buf, void *tag);
+ void arc_buf_info(arc_buf_t *buf, arc_buf_info_t *abi, int state_index);
+ uint64_t arc_buf_size(arc_buf_t *buf);
+ uint64_t arc_buf_lsize(arc_buf_t *buf);
++void arc_buf_access(arc_buf_t *buf);
+ void arc_release(arc_buf_t *buf, void *tag);
+ int arc_released(arc_buf_t *buf);
+ void arc_buf_sigsegv(int sig, siginfo_t *si, void *unused);
+diff --git a/module/zfs/arc.c b/module/zfs/arc.c
+index 2b0a78d4b..264e67735 100644
+--- a/module/zfs/arc.c
++++ b/module/zfs/arc.c
+@@ -429,9 +429,14 @@ typedef struct arc_stats {
+ * by multiple buffers.
+ */
+ kstat_named_t arcstat_mutex_miss;
++ /*
++ * Number of buffers skipped when updating the access state due to the
++ * header having already been released after acquiring the hash lock.
++ */
++ kstat_named_t arcstat_access_skip;
+ /*
+ * Number of buffers skipped because they have I/O in progress, are
+- * indrect prefetch buffers that have not lived long enough, or are
++ * indirect prefetch buffers that have not lived long enough, or are
+ * not from the spa we're trying to evict from.
+ */
+ kstat_named_t arcstat_evict_skip;
+@@ -667,6 +672,7 @@ static arc_stats_t arc_stats = {
+ { "mfu_ghost_hits", KSTAT_DATA_UINT64 },
+ { "deleted", KSTAT_DATA_UINT64 },
+ { "mutex_miss", KSTAT_DATA_UINT64 },
++ { "access_skip", KSTAT_DATA_UINT64 },
+ { "evict_skip", KSTAT_DATA_UINT64 },
+ { "evict_not_enough", KSTAT_DATA_UINT64 },
+ { "evict_l2_cached", KSTAT_DATA_UINT64 },
+@@ -4926,7 +4932,51 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
+ }
+ }
+
+-/* a generic arc_done_func_t which you can use */
++/*
++ * This routine is called by dbuf_hold() to update the arc_access() state
++ * which otherwise would be skipped for entries in the dbuf cache.
++ */
++void
++arc_buf_access(arc_buf_t *buf)
++{
++ mutex_enter(&buf->b_evict_lock);
++ arc_buf_hdr_t *hdr = buf->b_hdr;
++
++ /*
++ * Avoid taking the hash_lock when possible as an optimization.
++ * The header must be checked again under the hash_lock in order
++ * to handle the case where it is concurrently being released.
++ */
++ if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
++ mutex_exit(&buf->b_evict_lock);
++ return;
++ }
++
++ kmutex_t *hash_lock = HDR_LOCK(hdr);
++ mutex_enter(hash_lock);
++
++ if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
++ mutex_exit(hash_lock);
++ mutex_exit(&buf->b_evict_lock);
++ ARCSTAT_BUMP(arcstat_access_skip);
++ return;
++ }
++
++ mutex_exit(&buf->b_evict_lock);
++
++ ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
++ hdr->b_l1hdr.b_state == arc_mfu);
++
++ DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
++ arc_access(hdr, hash_lock);
++ mutex_exit(hash_lock);
++
++ ARCSTAT_BUMP(arcstat_hits);
++ ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr), demand, prefetch,
++ !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
++}
++
++/* a generic arc_read_done_func_t which you can use */
+ /* ARGSUSED */
+ void
+ arc_bcopy_func(zio_t *zio, arc_buf_t *buf, void *arg)
+diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c
+index 60f52d294..4ee121f5a 100644
+--- a/module/zfs/dbuf.c
++++ b/module/zfs/dbuf.c
+@@ -2719,8 +2719,10 @@ __dbuf_hold_impl(struct dbuf_hold_impl_data *dh)
+ return (SET_ERROR(ENOENT));
+ }
+
+- if (dh->dh_db->db_buf != NULL)
++ if (dh->dh_db->db_buf != NULL) {
++ arc_buf_access(dh->dh_db->db_buf);
+ ASSERT3P(dh->dh_db->db.db_data, ==, dh->dh_db->db_buf->b_data);
++ }
+
+ ASSERT(dh->dh_db->db_buf == NULL || arc_referenced(dh->dh_db->db_buf));
+
+--
+2.14.2
+
projects / zfsonlinux.git / commitdiff