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Optimize pathologic case of telldir() for Samba.

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When application reads large directory, calling telldir() for each entry,
like Samba does, it creates exponential performance drop as number of
entries reach tenths to hundreds of thousands.  It is caused by full search
through the internal list, that never finds matches in that scenario, but
creates O(n^2) delays.  This patch optimizes that search, limiting it to
entries of the same buffer, turning time closer to O(n) in case of linear
directory scan.

PR:		218622
Reviewed by:	jhb, jilles
MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D10408
This commit is contained in:
Alexander Motin 2017-04-17 19:03:31 +00:00
parent f0e56c1f62
commit f924326575
1 changed files with 14 additions and 4 deletions
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18
lib/libc/gen/telldir.c
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@ -52,15 +52,22 @@ __FBSDID("$FreeBSD$");
long
telldir(DIR *dirp)
{
struct ddloc *lp;
struct ddloc *lp, *flp;
long idx;
if (__isthreaded)
_pthread_mutex_lock(&dirp->dd_lock);
flp = NULL;
LIST_FOREACH(lp, &dirp->dd_td->td_locq, loc_lqe) {
if (lp->loc_seek == dirp->dd_seek &&
lp->loc_loc == dirp->dd_loc)
if (lp->loc_seek == dirp->dd_seek) {
if (flp == NULL)
flp = lp;
if (lp->loc_loc == dirp->dd_loc)
break;
} else if (flp != NULL) {
lp = NULL;
break;
}
}
if (lp == NULL) {
lp = malloc(sizeof(struct ddloc));
@ -72,7 +79,10 @@ telldir(DIR *dirp)
lp->loc_index = dirp->dd_td->td_loccnt++;
lp->loc_seek = dirp->dd_seek;
lp->loc_loc = dirp->dd_loc;
LIST_INSERT_HEAD(&dirp->dd_td->td_locq, lp, loc_lqe);
if (flp != NULL)
LIST_INSERT_BEFORE(flp, lp, loc_lqe);
else
LIST_INSERT_HEAD(&dirp->dd_td->td_locq, lp, loc_lqe);
}
idx = lp->loc_index;
if (__isthreaded)