Messages in this thread | | | Date | Fri, 18 Feb 2011 12:22:03 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] mm: vmscan: Stop reclaim/compaction earlier due to insufficient progress if !__GFP_REPEAT |
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On Thu, Feb 17, 2011 at 02:22:09PM -0800, Andrew Morton wrote: > On Wed, 16 Feb 2011 09:50:49 +0000 > Mel Gorman <mel@csn.ul.ie> wrote: > > > should_continue_reclaim() for reclaim/compaction allows scanning to continue > > even if pages are not being reclaimed until the full list is scanned. In > > terms of allocation success, this makes sense but potentially it introduces > > unwanted latency for high-order allocations such as transparent hugepages > > and network jumbo frames that would prefer to fail the allocation attempt > > and fallback to order-0 pages. Worse, there is a potential that the full > > LRU scan will clear all the young bits, distort page aging information and > > potentially push pages into swap that would have otherwise remained resident. > > afaict the patch affects order-0 allocations as well. What are the > implications of this? >
order-0 allocation should not be affected because RECLAIM_MODE_COMPACTION is not set so the following avoids the gfp_mask being examined;
if (!(sc->reclaim_mode & RECLAIM_MODE_COMPACTION)) return false;
> Also, what might be the downsides of this change, and did you test for > them? >
The main downside that I predict is that the worst-case latencies for successful transparent hugepage allocations will be increased as there will be more looping in do_try_to_free_pages() at higher priorities. I would also not be surprised if there were fewer successful allocations.
Latencies did seem to be worse for order-9 allocations in testing but it was offset by lower latencies for lower orders and seemed an acceptable trade-off.
Other major consequences did not spring to mind.
> > This patch will stop reclaim/compaction if no pages were reclaimed in the > > last SWAP_CLUSTER_MAX pages that were considered. > > a) Why SWAP_CLUSTER_MAX? Is (SWAP_CLUSTER_MAX+7) better or worse? >
SWAP_CLUSTER_MAX is the standard "unit of reclaim" and that's what I had in mind when writing the comment but it's wrong and misleading. More on this below.
> b) The sentence doesn't seem even vaguely accurate. shrink_zone() > will scan vastly more than SWAP_CLUSTER_MAX pages before calling > should_continue_reclaim(). Confused. > > c) The patch doesn't "stop reclaim/compaction" fully. It stops it > against one zone. reclaim will then advance on to any other > eligible zones.
You're right on both counts and this comment is inaccurate. It should have read;
This patch will stop reclaim/compaction for the current zone in shrink_zone() if there were no pages reclaimed in the last batch of scanning at the current priority. For allocations such as hugetlbfs that use __GFP_REPEAT and have fewer fallback options, the full LRU list may still be scanned.
The comment in the code itself then becomes
+ /* + * For non-__GFP_REPEAT allocations which can presumably + * fail without consequence, stop if we failed to reclaim + * any pages from the last batch of pages that were scanned. + * This will return to the caller faster at the risk that + * reclaim/compaction and the resulting allocation attempt + * fails + */
-- Mel Gorman SUSE Labs
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