Re: [RFC/PATCH] Optimize zone allocator synchronization

[Andrew Morton <akpm@linux-foundation.org>]

> On Tue, 06 Nov 2007 05:08:07 -0500 Chris Snook <csnook@redhat.com> wrote:
> Don Porter wrote:
> > From: Donald E. Porter <porterde@cs.utexas.edu>
> > 
> > In the bulk page allocation/free routines in mm/page_alloc.c, the zone
> > lock is held across all iterations.  For certain parallel workloads, I
> > have found that releasing and reacquiring the lock for each iteration
> > yields better performance, especially at higher CPU counts.  For
> > instance, kernel compilation is sped up by 5% on an 8 CPU test
> > machine.  In most cases, there is no significant effect on performance
> > (although the effect tends to be slightly positive).  This seems quite
> > reasonable for the very small scope of the change.
> > 
> > My intuition is that this patch prevents smaller requests from waiting
> > on larger ones.  While grabbing and releasing the lock within the loop
> > adds a few instructions, it can lower the latency for a particular
> > thread's allocation which is often on the thread's critical path.
> > Lowering the average latency for allocation can increase system throughput.
> > 
> > More detailed information, including data from the tests I ran to
> > validate this change are available at
> > http://www.cs.utexas.edu/~porterde/kernel-patch.html .
> > 
> > Thanks in advance for your consideration and feedback.
> 
> That's an interesting insight.  My intuition is that Nick Piggin's 
> recently-posted ticket spinlocks patches[1] will reduce the need for this patch, 
> though it may be useful to have both.  Can you benchmark again with only ticket 
> spinlocks, and with ticket spinlocks + this patch?  You'll probably want to use 
> 2.6.24-rc1 as your baseline, due to the x86 architecture merge.

The patch as-is would hurt low cpu-count workloads, and single-threaded
workloads: it is simply taking that lock a lot more times.  This will be
particuarly noticable on things like older P4 machines which have peculiarly
expensive locked operations.

A test to run would be, on ext2:

	time (dd if=/dev/zero of=foo bs=16k count=2048 ; rm foo)

(might need to increase /proc/sys/vm/dirty* to avoid any writeback)


I wonder if we can do something like:

	if (lock_is_contended(lock)) {
		spin_unlock(lock);
		spin_lock(lock);		/* To the back of the queue */
	}

(in conjunction with the ticket locks) so that we only do the expensive
buslocked operation when we actually have a need to do so.

(The above should be wrapped in some new spinlock interface function which
is probably a no-op on architectures which cannot implement it usefully)