Messages in this thread | | | Date | Tue, 6 Nov 2007 22:19:28 -0800 | From | Andrew Morton <> | Subject | Re: [RFC/PATCH] Optimize zone allocator synchronization |
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> 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) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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