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cache: ignore purgevfs requests for filesystems with few vnodes

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purgevfs is purely optional and induces lock contention in workloads
which frequently mount and unmount filesystems.

In particular, poudriere will do this for filesystems with 4 vnodes or
less. Full cache scan is clearly wasteful.

Since there is no explicit counter for namecache entries, the number of
vnodes used by the target fs is checked.

The default limit is the number of bucket locks.

Reviewed by:	kib
This commit is contained in:
Mateusz Guzik 2016-10-03 00:02:32 +00:00
parent b78bd5ec30
commit 4876636eb7
1 changed files with 6 additions and 0 deletions
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6
sys/kern/vfs_cache.c
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@ -207,6 +207,9 @@ SYSCTL_ULONG(_debug, OID_AUTO, numcachehv, CTLFLAG_RD, &numcachehv, 0,
u_int ncsizefactor = 2;
SYSCTL_UINT(_vfs, OID_AUTO, ncsizefactor, CTLFLAG_RW, &ncsizefactor, 0,
"Size factor for namecache");
static u_int ncpurgeminvnodes;
SYSCTL_UINT(_vfs, OID_AUTO, ncpurgeminvnodes, CTLFLAG_RW, &ncpurgeminvnodes, 0,
"Number of vnodes below which purgevfs ignores the request");
struct nchstats nchstats; /* cache effectiveness statistics */
@ -1614,6 +1617,7 @@ nchinit(void *dummy __unused)
M_WAITOK | M_ZERO);
for (i = 0; i < numvnodelocks; i++)
mtx_init(&vnodelocks[i], "ncvn", NULL, MTX_DUPOK | MTX_RECURSE);
ncpurgeminvnodes = numbucketlocks;
numcalls = counter_u64_alloc(M_WAITOK);
dothits = counter_u64_alloc(M_WAITOK);
@ -1764,6 +1768,8 @@ cache_purgevfs(struct mount *mp)
/* Scan hash tables for applicable entries */
SDT_PROBE1(vfs, namecache, purgevfs, done, mp);
if (mp->mnt_nvnodelistsize <= ncpurgeminvnodes)
return;
TAILQ_INIT(&ncps);
n_nchash = nchash + 1;
vlp1 = vlp2 = NULL;