Re: [PATCH 7/7] mm: compaction: Introduce sync-light migration foruse by compaction

[Nai Xia]

On Tue, Nov 22, 2011 at 2:56 PM, Shaohua Li <shaohua.li@xxxxxxxxx> 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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