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@xxxxxxxxx> 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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