TDB locking overhead and performance...
Daniel J Blueman
daniel.blueman at gmail.com
Sun Apr 23 14:32:56 GMT 2006
On 22/04/06, David Collier-Brown <davec-b at rogers.com> wrote:
> Linux indeed hs some fancy locks for this, which were tested
> with TDBs (see
> but there is no discussion of what happens when we lose a
> process which is holding a mmap'd lock. That was explicitly
> not addressed in the paper, from the Ottawa Linux Symposium.
> This may have been addressed by now, but I don't see any
> references to it.
> Fcntl locks explicitly release if the locking process had
> died, so deadlock isn't a big issue. For that reason they're
> preferred, even though they're far slower than spinlocks
> on Solaris (that's a Solaris bug, by the way).
I was looking at futexes, but I found that current glibc packages
don't seem to provide a user-space header file (glibc seems to use
futexes internally). On top of this, we still need atomic operations,
which glibc doesn't provide to user-space.
glibc's pthreads use futexes (see PTHREAD_MUTEX_FAST_NP) and this
seems to work across processes sharing the lock, however the same
problem is there, that when a process holding the lock gets killed,
the lock isn't released. The recent robust futex work should deal with
this, but will be some time before it hits kernel + glibc.
Does anyone know if sysV IPC semaphores have any mechanism for recovering locks?
One mechanism to handle the process-crashing-holding-lock corner case
is to have lock acquisition timeout after eg 10s, and check if the pid
of the locker is still there - if not, the lock is dropped by 1.
Any comments on why this is a bad/unworkable idea with eg sysV semaphores?
> Volker Lendecke wrote:
> > On Sat, Apr 22, 2006 at 06:15:31PM +0100, Daniel J Blueman wrote:
> >>As a test, I changed the opening of the locking.tdb and brlock.tdb
> >>files to use the TDB_INTERNAL flag, avoiding use of the fcntl(F_SETLK)
> >>syscall for locking individual database records. Performance was a lot
> >>snappier, with quite a bit less system time used.
> >>What is the scope of implementing shared memory TDBs, where locking
> >>could be done simply on structures?
> > How would you implement cross-process locking without
> > any syscalls? Maybe Linux has some fancy stuff here, but for
> > most unixes we need to coordinate the mmap'ed area for the
> > tdbs fcntl locks.
> > Volker
Daniel J Blueman
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