Re: Performance regression from switching lock to rw-sem foranon-vma tree

From: Tim Chen
Date: Mon Aug 05 2013 - 18:08:29 EST

On Tue, 2013-07-30 at 21:24 +0200, Ingo Molnar wrote:

> >
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > + bool "Optimistic spin write acquisition for writer owned rw-sem"
> > + default n
> > + depends on SMP
> > + help
> > + Allows a writer to perform optimistic spinning if another writer own
> > + the read write semaphore. If the lock owner is running, it is likely
> > + to release the lock soon. Spinning gives a greater chance for writer to
> > + acquire a semaphore before putting it to sleep.
> The way you've worded this new Kconfig option makes it
> sound as if it's not just for testing, and I'm not a big
> believer in extra Kconfig degrees of freedom for
> scalability features of core locking primitives like
> rwsems, in production kernels ...
> So the bad news is that such scalability optimizations
> really need to work for everyone.
> The good news is that I don't think there's anything
> particularly controversial about making the rwsem write
> side perform just as well as mutexes - it would in fact be
> a very nice quality of implementation feature: it gives
> people freedom to switch between mutexes and rwsems without
> having to worry about scalability differences too much.

Sorry for replying to your email late as I was pulled to
some other tasks.

Ingo, any objection if I make the optimistic writer spin the
default for SMP without an extra config? This will make
the rw_semaphore structure grow a bit to accommodate the
owner and spin_mlock field.


> Once readers are mixed into the workload can we keep the
> XFS assumptions, if they are broken in any way?
> We are spinning here so we have full awareness about the
> state of the lock and we can react to a new reader in very
> short order - so at a quick glance I don't see any
> fundamental difficulty in being able to resolve it - beyond
> the SMOP aspect that is ... :-)
> Thanks,
> Ingo

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