Date: Sat, 7 Jul 2007 14:26:17 +0100 From: Doug Rabson <dfr@rabson.org> To: current@freebsd.org Subject: ZFS leaking vnodes (sort of) Message-ID: <200707071426.18202.dfr@rabson.org>
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I've been testing ZFS recently and I noticed some performance issues
while doing large-scale port builds on a ZFS mounted /usr/ports tree.
Eventually I realised that virtually nothing ever ended up on the vnode
free list. This meant that when the system reached its maximum vnode
limit, it had to resort to reclaiming vnodes from the various
filesystem's active vnode lists (via vlrureclaim). Since those lists
are not sorted in LRU order, this led to pessimal cache performance
after the system got into that state.
I looked a bit closer at the ZFS code and poked around with DDB and I
think the problem was caused by a couple of extraneous calls to vhold
when creating a new ZFS vnode. On FreeBSD, getnewvnode returns a vnode
which is already held (not on the free list) so there is no need to
call vhold again.
This patch appears to fix the problem (only very lightly tested):
Index: zfs_vnops.c
===================================================================
RCS
file: /home/ncvs/src/sys/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c,v
retrieving revision 1.22
diff -u -r1.22 zfs_vnops.c
--- zfs_vnops.c 28 May 2007 02:37:43 -0000 1.22
+++ zfs_vnops.c 7 Jul 2007 13:01:41 -0000
@@ -3493,7 +3493,7 @@
rele = 0;
vp->v_data = NULL;
ASSERT(vp->v_holdcnt > 1);
- vdropl(vp);
+ VI_UNLOCK(vp);
if (!zp->z_unlinked && rele)
VFS_RELE(zfsvfs->z_vfs);
return (0);
Index: zfs_znode.c
===================================================================
RCS
file: /home/ncvs/src/sys/contrib/opensolaris/uts/common/fs/zfs/zfs_znode.c,v
retrieving revision 1.8
diff -u -r1.8 zfs_znode.c
--- zfs_znode.c 6 May 2007 19:05:37 -0000 1.8
+++ zfs_znode.c 7 Jul 2007 13:17:32 -0000
@@ -115,7 +115,6 @@
ASSERT(error == 0);
zp->z_vnode = vp;
vp->v_data = (caddr_t)zp;
- vhold(vp);
vp->v_vnlock->lk_flags |= LK_CANRECURSE;
vp->v_vnlock->lk_flags &= ~LK_NOSHARE;
} else {
@@ -601,7 +600,6 @@
ASSERT(err == 0);
vp = ZTOV(zp);
vp->v_data = (caddr_t)zp;
- vhold(vp);
vp->v_vnlock->lk_flags |= LK_CANRECURSE;
vp->v_vnlock->lk_flags &= ~LK_NOSHARE;
vp->v_type = IFTOVT((mode_t)zp->z_phys->zp_mode);
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