Re: [RFC/PATCH] Optimize zone allocator synchronization

[Chris Snook <csnook@redhat.com>]

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.

	-- Chris

[1] http://lkml.org/lkml/2007/11/1/123
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