Messages in this thread | | | Subject | Re: [RFC PATCH 00/20] Cleanup and optimise the page allocator | From | Andi Kleen <> | Date | Mon, 23 Feb 2009 00:57:37 +0100 |
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Mel Gorman <mel@csn.ul.ie> writes:
> The complexity of the page allocator has been increasing for some time > and it has now reached the point where the SLUB allocator is doing strange > tricks to avoid the page allocator. This is obviously bad as it may encourage > other subsystems to try avoiding the page allocator as well.
Congratulations! That was long overdue. Haven't read the patches yet though.
> Patch 15 reduces the number of times interrupts are disabled by reworking > what free_page_mlock() does. However, I notice that the cost of calling > TestClearPageMlocked() is still quite high and I'm guessing it's because > it's a locked bit operation. It's be nice if it could be established if > it's safe to use an unlocked version here. Rik, can you comment?
What machine was that again?
> Patch 16 avoids using the zonelist cache on non-NUMA machines
My suspicion is that it can be even dropped on most small (all?) NUMA systems.
> Patch 20 gets rid of hot/cold freeing of pages because it incurs cost for > what I believe to be very dubious gain. I'm not sure we currently gain > anything by it but it's further discussed in the patch itself.
Yes the hot/cold thing was always quite dubious.
> Counters are surprising expensive, we spent a good chuck of our time in > functions like __dec_zone_page_state and __dec_zone_state. In a profiled > run of kernbench, the time spent in __dec_zone_state was roughly equal to > the combined cost of the rest of the page free path. A quick check showed > that almost half of the time in that function is spent on line 233 alone > which for me is; > > (*p)--; > > That's worth a separate investigation but it might be a case that > manipulating int8_t on the machine I was using for profiling is unusually > expensive.
What machine was that?
In general I wouldn't expect even on a system with slow char operations to be that expensive. It sounds more like a cache miss or a cache line bounce. You could possibly confirm by using appropiate performance counters.
> Converting this to an int might be faster but the increased > memory consumption and cache footprint might be a problem. Opinions?
One possibility would be to move the zone statistics to allocated per cpu data. Or perhaps just stop counting per zone at all and only count per cpu.
> The downside is that the patches do increase text size because of the > splitting of the fast path into one inlined blob and the slow path into a > number of other functions. On my test machine, text increased by 1.2K so > I might revisit that again and see how much of a difference it really made. > > That all said, I'm seeing good results on actual benchmarks with these > patches. > > o On many machines, I'm seeing a 0-2% improvement on kernbench. The dominant
Neat.
> So, by and large it's an improvement of some sort.
That seems like an understatement.
-Andi -- ak@linux.intel.com -- Speaking for myself only.
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