Author: mjg
Date: Fri Sep 27 19:13:22 2019
New Revision: 352812
URL: https://svnweb.freebsd.org/changeset/base/352812
Log:
cache: stop requeuing negative entries on the hot list
Turns out it does not improve hit ratio, but it does come with a cost
induces stemming from dirtying hit entries.
Sample result: hit counts of evicted entries after 2 buildworlds
before:
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@@@@@@@@@@@@@@@@@ 180865
1 |@@@@@@@ 49150
2 |@@@ 19067
4 |@ 9825
8 |@ 7340
16 |@ 5952
32 |@ 5243
64 |@ 4446
128 | 3556
256 | 3035
512 | 1705
1024 | 1078
2048 | 365
4096 | 95
8192 | 34
16384 | 26
32768 | 23
65536 | 8
131072 | 6
262144 | 0
after:
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@@@@@@@@@@@@@@@@@ 184004
1 |@@@@@@ 47577
2 |@@@ 19446
4 |@ 10093
8 |@ 7470
16 |@ 5544
32 |@ 5475
64 |@ 5011
128 | 3451
256 | 3002
512 | 1729
1024 | 1086
2048 | 363
4096 | 86
8192 | 26
16384 | 25
32768 | 24
65536 | 7
131072 | 5
262144 | 0
Sponsored by: The FreeBSD Foundation
Modified:
head/sys/kern/vfs_cache.c
Modified: head/sys/kern/vfs_cache.c
==============================================================================
--- head/sys/kern/vfs_cache.c Fri Sep 27 19:12:43 2019 (r352811)
+++ head/sys/kern/vfs_cache.c Fri Sep 27 19:13:22 2019 (r352812)
@@ -92,10 +92,10 @@ SDT_PROBE_DEFINE1(vfs, namecache, purge_negative, done
SDT_PROBE_DEFINE1(vfs, namecache, purgevfs, done, "struct mount *");
SDT_PROBE_DEFINE3(vfs, namecache, zap, done, "struct vnode *", "char *",
"struct vnode *");
-SDT_PROBE_DEFINE3(vfs, namecache, zap_negative, done, "struct vnode *",
- "char *", "int");
-SDT_PROBE_DEFINE3(vfs, namecache, shrink_negative, done, "struct vnode *",
- "char *", "int");
+SDT_PROBE_DEFINE2(vfs, namecache, zap_negative, done, "struct vnode *",
+ "char *");
+SDT_PROBE_DEFINE2(vfs, namecache, shrink_negative, done, "struct vnode *",
+ "char *");
/*
* This structure describes the elements in the cache of recent
@@ -109,7 +109,6 @@ struct namecache {
struct vnode *nc_dvp; /* vnode of parent of name */
union {
struct vnode *nu_vp; /* vnode the name refers to */
- u_int nu_neghits; /* negative entry hits */
} n_un;
u_char nc_flag; /* flag bits */
u_char nc_nlen; /* length of name */
@@ -132,7 +131,6 @@ struct namecache_ts {
};
#define nc_vp n_un.nu_vp
-#define nc_neghits n_un.nu_neghits
/*
* Flags in namecache.nc_flag
@@ -220,9 +218,6 @@ SYSCTL_UINT(_vfs, OID_AUTO, ncsizefactor, CTLFLAG_RW,
static u_int __read_mostly ncpurgeminvnodes;
SYSCTL_UINT(_vfs, OID_AUTO, ncpurgeminvnodes, CTLFLAG_RW, &ncpurgeminvnodes, 0,
"Number of vnodes below which purgevfs ignores the request");
-static u_int __read_mostly ncneghitsrequeue = 8;
-SYSCTL_UINT(_vfs, OID_AUTO, ncneghitsrequeue, CTLFLAG_RW, &ncneghitsrequeue, 0,
- "Number of hits to requeue a negative entry in the LRU list");
static u_int __read_mostly ncsize; /* the size as computed on creation or
resizing */
struct nchstats nchstats; /* cache effectiveness
statistics */
@@ -676,8 +671,6 @@ SYSCTL_PROC(_debug_hashstat, OID_AUTO, nchash, CTLTYPE
*
* A variation of LRU scheme is used. New entries are hashed into one of
* numneglists cold lists. Entries get promoted to the hot list on first hit.
- * Partial LRU for the hot list is maintained by requeueing them every
- * ncneghitsrequeue hits.
*
* The shrinker will demote hot list head and evict from the cold list in a
* round-robin manner.
@@ -686,28 +679,12 @@ static void
cache_negative_hit(struct namecache *ncp)
{
struct neglist *neglist;
- u_int hits;
MPASS(ncp->nc_flag & NCF_NEGATIVE);
- hits = atomic_fetchadd_int(&ncp->nc_neghits, 1);
- if (ncp->nc_flag & NCF_HOTNEGATIVE) {
- if ((hits % ncneghitsrequeue) != 0)
- return;
- mtx_lock(&ncneg_hot.nl_lock);
- if (ncp->nc_flag & NCF_HOTNEGATIVE) {
- TAILQ_REMOVE(&ncneg_hot.nl_list, ncp, nc_dst);
- TAILQ_INSERT_TAIL(&ncneg_hot.nl_list, ncp, nc_dst);
- mtx_unlock(&ncneg_hot.nl_lock);
- return;
- }
- /*
- * The shrinker cleared the flag and removed the entry from
- * the hot list. Put it back.
- */
- } else {
- mtx_lock(&ncneg_hot.nl_lock);
- }
+ if (ncp->nc_flag & NCF_HOTNEGATIVE)
+ return;
neglist = NCP2NEGLIST(ncp);
+ mtx_lock(&ncneg_hot.nl_lock);
mtx_lock(&neglist->nl_lock);
if (!(ncp->nc_flag & NCF_HOTNEGATIVE)) {
numhotneg++;
@@ -851,8 +828,9 @@ cache_negative_zap_one(void)
blp != NCP2BUCKETLOCK(ncp2) || !(ncp2->nc_flag & NCF_NEGATIVE)) {
ncp = NULL;
} else {
- SDT_PROBE3(vfs, namecache, shrink_negative, done, ncp->nc_dvp,
- ncp->nc_name, ncp->nc_neghits);
+ SDT_PROBE2(vfs, namecache, shrink_negative, done, ncp->nc_dvp,
+ ncp->nc_name);
+
cache_zap_locked(ncp, true);
counter_u64_add(numneg_evicted, 1);
}
@@ -887,8 +865,8 @@ cache_zap_locked(struct namecache *ncp, bool neg_locke
if (ncp == ncp->nc_vp->v_cache_dd)
ncp->nc_vp->v_cache_dd = NULL;
} else {
- SDT_PROBE3(vfs, namecache, zap_negative, done, ncp->nc_dvp,
- ncp->nc_name, ncp->nc_neghits);
+ SDT_PROBE2(vfs, namecache, zap_negative, done, ncp->nc_dvp,
+ ncp->nc_name);
cache_negative_remove(ncp, neg_locked);
}
if (ncp->nc_flag & NCF_ISDOTDOT) {
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