Messages in this thread | | | Date | Mon, 25 Jul 2011 23:01:48 +0200 | From | Andrea Arcangeli <> | Subject | Re: [patch] mm: thp: disable defrag for page faults per default |
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Hello Johannes,
On Mon, Jul 25, 2011 at 10:38:41PM +0200, Johannes Weiner wrote: > With defrag mode enabled per default, huge page allocations pass > __GFP_WAIT and may drop compaction into sync-mode where they wait for > pages under writeback. > > I observe applications hang for several minutes(!) when they fault in > huge pages and compaction starts to wait on in-"flight" USB stick IO. > > This patch disables defrag mode for page fault allocations unless the > VMA is madvised explicitely. Khugepaged will continue to allocate > with __GFP_WAIT per default, but stalls are not a problem of > application responsiveness there.
Allocating memory without __GFP_WAIT means THP it's like disabled except when there's plenty of memory free after boot, even trying with __GFP_WAIT and without compaction would be better than that. We don't want to modify all apps, just a few special ones should have the madvise like qemu-kvm for example (for embedded in case there's embedded virt).
If you want to make compaction and migrate run without ever dropping into sync-mode (or aborting if we've to wait on too many pages) I think it'd be a whole lot better.
If you could show the SYSRQ+T during the minute wait it'd be interesting too.
There was also some compaction bug that would lead to minutes of stall in congestion_wait, those are fixed in current kernels.
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