Date: Mon, 12 Jun 2006 01:41:15 -0400 From: Kris Kennaway <kris@obsecurity.org> To: current@FreeBSD.org Subject: FILEDESC_LOCK() implementation Message-ID: <20060612054115.GA42379@xor.obsecurity.org>
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I fixed mutex profiling to a) not be as wrong and b) not suck so very
much, and here is a revised profiling trace from mysql supersmack on a
12 cpu E4500, sorted by ratio of cnt_lock/count; filedesc lock
contention (via FILEDESC_[UN]LOCK()) is the major mutex contention
problem.
0 0 55381 0 672 3195 .057 kern/kern_sig.c:998 (process lock)
0 0 31451 0 3657 2446 .077 kern/subr_sleepqueue.c:374 (process lock)
0 0 31706 0 3570 2623 .082 kern/kern_sx.c:245 (lockbuilder mtxpool)
0 0 458877 0 83592 48024 .104 kern/kern_descrip.c:1988 (filedesc structure)
0 0 45069 0 5342 6085 .135 kern/kern_umtx.c:195 (umtxq_lock)
0 0 458877 0 22609 72747 .158 kern/kern_descrip.c:1967 (filedesc structure)
0 0 40046 0 6845 6925 .172 kern/uipc_usrreq.c:621 (unp_global_mtx)
0 0 31706 0 3159 6075 .191 kern/kern_sx.c:157 (lockbuilder mtxpool)
0 0 66539 0 22823 12844 .193 kern/kern_descrip.c:420 (filedesc structure)
0 0 66569 0 5352 20913 .314 kern/kern_descrip.c:368 (filedesc structure)
0 0 110665 0 38632 76441 .690 kern/kern_synch.c:222 (filedesc structure)
#
kern/kern_descrip.c:1967,1988 are _fget()
kern/kern_descrip.c:368,420 is kern_fcntl()
kern/kern_synch.c:222 is msleep() (called in a loop from FILEDESC_LOCK)
I wonder if something better can be done with the funky home-grown
locking in FILEDESC_LOCK() (see <sys/filedesc.h>) to make it more
light-weight?
/* Lock a file descriptor table. */
#define FILEDESC_LOCK(fd) \
do { \
mtx_lock(&(fd)->fd_mtx); \
(fd)->fd_wanted++; \
while ((fd)->fd_locked) \
msleep(&(fd)->fd_locked, &(fd)->fd_mtx, PLOCK, "fdesc", 0); \
(fd)->fd_locked = 2; \
(fd)->fd_wanted--; \
mtx_unlock(&(fd)->fd_mtx); \
} while (0)
Kris
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