On 01/24/2014 05:03 PM, Johannes Weiner wrote:

The VM is currently heavily tuned to avoid swapping.  Whether that is
good or bad is a separate discussion, but as long as the VM won't swap
to make room for dirty cache, we can not consider anonymous pages when
calculating the amount of dirtyable memory, the baseline to which
dirty_background_ratio and dirty_ratio are applied.

A simple workload that occupies a significant size (40+%, depending on
memory layout, storage speeds etc.) of memory with anon/tmpfs pages
and uses the remainder for a streaming writer demonstrates this
problem.  In that case, the actual cache pages are a small fraction of
what is considered dirtyable overall, which results in an relatively
large portion of the cache pages to be dirtied.  As kswapd starts
rotating these, random tasks enter direct reclaim and stall on IO.

Only consider free pages and file pages dirtyable.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Rik van Riel <redacted>


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