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cache: stop requeuing negative entries on the hot list

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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
This commit is contained in:
mjg 2019-09-27 19:13:22 +00:00
parent a67ed57908
commit 306794b4a0
1 changed files with 12 additions and 34 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

46
sys/kern/vfs_cache.c
View File

@ -92,10 +92,10 @@ SDT_PROBE_DEFINE1(vfs, namecache, purge_negative, done, "struct vnode *");
SDT_PROBE_DEFINE1(vfs, namecache, purgevfs, done, "struct mount *"); SDT_PROBE_DEFINE1(vfs, namecache, purgevfs, done, "struct mount *");
SDT_PROBE_DEFINE3(vfs, namecache, zap, done, "struct vnode *", "char *", SDT_PROBE_DEFINE3(vfs, namecache, zap, done, "struct vnode *", "char *",
"struct vnode *"); "struct vnode *");
SDT_PROBE_DEFINE3(vfs, namecache, zap_negative, done, "struct vnode *", SDT_PROBE_DEFINE2(vfs, namecache, zap_negative, done, "struct vnode *",
"char *", "int"); "char *");
SDT_PROBE_DEFINE3(vfs, namecache, shrink_negative, done, "struct vnode *", SDT_PROBE_DEFINE2(vfs, namecache, shrink_negative, done, "struct vnode *",
"char *", "int"); "char *");
/* /*
* This structure describes the elements in the cache of recent * 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 */ struct vnode *nc_dvp; /* vnode of parent of name */
union { union {
struct vnode *nu_vp; /* vnode the name refers to */ struct vnode *nu_vp; /* vnode the name refers to */
u_int nu_neghits; /* negative entry hits */
} n_un; } n_un;
u_char nc_flag; /* flag bits */ u_char nc_flag; /* flag bits */
u_char nc_nlen; /* length of name */ u_char nc_nlen; /* length of name */
@ -132,7 +131,6 @@ struct namecache_ts {
}; };
#define nc_vp n_un.nu_vp #define nc_vp n_un.nu_vp
#define nc_neghits n_un.nu_neghits
/* /*
* Flags in namecache.nc_flag * Flags in namecache.nc_flag
@ -220,9 +218,6 @@ SYSCTL_UINT(_vfs, OID_AUTO, ncsizefactor, CTLFLAG_RW, &ncsizefactor, 0,
static u_int __read_mostly ncpurgeminvnodes; static u_int __read_mostly ncpurgeminvnodes;
SYSCTL_UINT(_vfs, OID_AUTO, ncpurgeminvnodes, CTLFLAG_RW, &ncpurgeminvnodes, 0, SYSCTL_UINT(_vfs, OID_AUTO, ncpurgeminvnodes, CTLFLAG_RW, &ncpurgeminvnodes, 0,
"Number of vnodes below which purgevfs ignores the request"); "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 */ static u_int __read_mostly ncsize; /* the size as computed on creation or resizing */
struct nchstats nchstats; /* cache effectiveness statistics */ struct nchstats nchstats; /* cache effectiveness statistics */
@ -676,8 +671,6 @@ SYSCTL_PROC(_debug_hashstat, OID_AUTO, nchash, CTLTYPE_INT|CTLFLAG_RD|
* *
* A variation of LRU scheme is used. New entries are hashed into one of * 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. * 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 * The shrinker will demote hot list head and evict from the cold list in a
* round-robin manner. * round-robin manner.
@ -686,28 +679,12 @@ static void
cache_negative_hit(struct namecache *ncp) cache_negative_hit(struct namecache *ncp)
{ {
struct neglist *neglist; struct neglist *neglist;
u_int hits;
MPASS(ncp->nc_flag & NCF_NEGATIVE); MPASS(ncp->nc_flag & NCF_NEGATIVE);
hits = atomic_fetchadd_int(&ncp->nc_neghits, 1); if (ncp->nc_flag & NCF_HOTNEGATIVE)
if (ncp->nc_flag & NCF_HOTNEGATIVE) { return;
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);
}
neglist = NCP2NEGLIST(ncp); neglist = NCP2NEGLIST(ncp);
mtx_lock(&ncneg_hot.nl_lock);
mtx_lock(&neglist->nl_lock); mtx_lock(&neglist->nl_lock);
if (!(ncp->nc_flag & NCF_HOTNEGATIVE)) { if (!(ncp->nc_flag & NCF_HOTNEGATIVE)) {
numhotneg++; numhotneg++;
@ -851,8 +828,9 @@ cache_negative_zap_one(void)
blp != NCP2BUCKETLOCK(ncp2) || !(ncp2->nc_flag & NCF_NEGATIVE)) { blp != NCP2BUCKETLOCK(ncp2) || !(ncp2->nc_flag & NCF_NEGATIVE)) {
ncp = NULL; ncp = NULL;
} else { } else {
SDT_PROBE3(vfs, namecache, shrink_negative, done, ncp->nc_dvp, SDT_PROBE2(vfs, namecache, shrink_negative, done, ncp->nc_dvp,
ncp->nc_name, ncp->nc_neghits); ncp->nc_name);
cache_zap_locked(ncp, true); cache_zap_locked(ncp, true);
counter_u64_add(numneg_evicted, 1); counter_u64_add(numneg_evicted, 1);
} }
@ -887,8 +865,8 @@ cache_zap_locked(struct namecache *ncp, bool neg_locked)
if (ncp == ncp->nc_vp->v_cache_dd) if (ncp == ncp->nc_vp->v_cache_dd)
ncp->nc_vp->v_cache_dd = NULL; ncp->nc_vp->v_cache_dd = NULL;
} else { } else {
SDT_PROBE3(vfs, namecache, zap_negative, done, ncp->nc_dvp, SDT_PROBE2(vfs, namecache, zap_negative, done, ncp->nc_dvp,
ncp->nc_name, ncp->nc_neghits); ncp->nc_name);
cache_negative_remove(ncp, neg_locked); cache_negative_remove(ncp, neg_locked);
} }
if (ncp->nc_flag & NCF_ISDOTDOT) { if (ncp->nc_flag & NCF_ISDOTDOT) {