Messages in this thread | | | Date | Wed, 23 Nov 2011 10:01:53 +0800 | Subject | Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration for use by compaction | From | Nai Xia <> |
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On Tue, Nov 22, 2011 at 2:56 PM, Shaohua Li <shaohua.li@intel.com> wrote: > On Tue, 2011-11-22 at 02:36 +0800, Mel Gorman wrote: >> This patch adds a lightweight sync migrate operation MIGRATE_SYNC_LIGHT >> mode that avoids writing back pages to backing storage. Async >> compaction maps to MIGRATE_ASYNC while sync compaction maps to >> MIGRATE_SYNC_LIGHT. For other migrate_pages users such as memory >> hotplug, MIGRATE_SYNC is used. >> >> This avoids sync compaction stalling for an excessive length of time, >> particularly when copying files to a USB stick where there might be >> a large number of dirty pages backed by a filesystem that does not >> support ->writepages. > Hi, > from my understanding, with this, even writes > to /proc/sys/vm/compact_memory doesn't wait for pageout, is this > intended? > on the other hand, MIGRATE_SYNC_LIGHT now waits for pagelock and buffer > lock, so could wait on page read. page read and page out have the same > latency, why takes them different?
So for the problem you raised, I think my suggestion to Mel is to adopt the following logic:
if (!trylock_page(page) && !PageUptodate(page)) we are quite likely to block on read, so we depend on yet another MIGRATE_SYNC_MODE to decide if we really want to lock_page() and wait for this IO.
How do you think ?
Thanks,
Nai > > Thanks, > Shaohua > >
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