[Opensim-dev] Lockless Lists?

[Hurliman, John]

-----Original Message-----
From: opensim-dev-bounces at lists.berlios.de [mailto:opensim-dev-bounces at lists.berlios.de] On Behalf Of Christopher Yeoh
Sent: Tuesday, November 25, 2008 3:26 PM
To: homer.horwitz at gmail.com; opensim-dev at lists.berlios.de
Subject: Re: [Opensim-dev] Lockless Lists?

On Mon, 24 Nov 2008 20:43:09 +0100
"Homer Horwitz" <homerhorwitz at googlemail.com> wrote:
>
> So, I'm not sure if we really should do that move. If at all, I'm for
> a very slow move to lock-free versions from a rather stable software
> base (which we currently don't have in trunk), so errors that are
> introduced during that move are more easily identifiable, with much
> testing in-between. Even then, I'm absolutely sure we will get a lot
> of Heisenbugs in the process, which will take us weeks to find.
>

Perhaps what is really needed here is some performance benchmarks
which highlight existing problems? So for individual
changes to lockless versions we better see what improvements we'd
get on both small and large SMP machines and whether its worth the
increase in complexity.

I did some debugging of a deadlock a couple of weeks ago and found it
already pretty complicated. Any suggestions on how other people
approach these problems with OpenSim? I sprinkled lots of console
messages around as mdb doesn't seem to work for me, but in retrospect
it would have been really handy to have been able to just turn on
a lock debugging flag and have debug output when locks are taken
and released.

Regards,

Chris



One idea I've tossed around is using Mono.Cecil to inject some code for profiling locks. Since

lock (lockObject)
{
	...
}

Gets expanded out by the compiler to:

Monitor.Enter(lockObject);
try
{
	...
}
finally
{
	Monitor.Exit(lockObject);
}

It shouldn't be too difficult to follow up any Monitor.Enter() calls by storing current ticks, and do a check in the finally if current ticks - stored ticks > lock_warn_const. Then we could scan any binary for long-held locks.

John