Messages in this thread | | | Date | Fri, 2 May 2014 11:22:31 +0100 | From | Mel Gorman <> | Subject | Re: [patch v2 4/4] mm, thp: do not perform sync compaction on pagefault |
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On Thu, May 01, 2014 at 02:35:48PM -0700, David Rientjes wrote: > Synchronous memory compaction can be very expensive: it can iterate an enormous > amount of memory without aborting, constantly rescheduling, waiting on page > locks and lru_lock, etc, if a pageblock cannot be defragmented. > > Unfortunately, it's too expensive for pagefault for transparent hugepages and > it's much better to simply fallback to pages. On 128GB machines, we find that > synchronous memory compaction can take O(seconds) for a single thp fault. > > Now that async compaction remembers where it left off without strictly relying > on sync compaction, this makes thp allocations best-effort without causing > egregious latency during pagefault. > > Signed-off-by: David Rientjes <rientjes@google.com>
Compaction uses MIGRATE_SYNC_LIGHT which in the current implementation avoids calling ->writepage to clean dirty page in the fallback migrate case
static int fallback_migrate_page(struct address_space *mapping, struct page *newpage, struct page *page, enum migrate_mode mode) { if (PageDirty(page)) { /* Only writeback pages in full synchronous migration */ if (mode != MIGRATE_SYNC) return -EBUSY; return writeout(mapping, page); }
or waiting on page writeback in other cases
/* * Only in the case of a full synchronous migration is it * necessary to wait for PageWriteback. In the async case, * the retry loop is too short and in the sync-light case, * the overhead of stalling is too much */ if (mode != MIGRATE_SYNC) { rc = -EBUSY; goto uncharge; }
or on acquiring the page lock for unforced migrations.
However, buffers still get locked in the SYNC_LIGHT case causing stalls in buffer_migrate_lock_buffers which may be undesirable or maybe you are hitting some other case. It would be preferable to identify what is getting stalled in SYNC_LIGHT compaction and fix that rather than disabling it entirely. You may also want to distinguish between a direct compaction by a process and collapsing huge pages as done by khugepaged.
-- Mel Gorman SUSE Labs
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