Re: [RFC 0/2] Use HighAtomic against long-term fragmentation

[Mel Gorman]

On Tue, Sep 26, 2017 at 04:46:42PM +0800, Hui Zhu wrote:
> Current HighAtomic just to handle the high atomic page alloc.
> But I found that use it handle the normal unmovable continuous page
> alloc will help to against long-term fragmentation.
> 

This is not wise. High-order atomic allocations do not always have a
smooth recovery path such as network drivers with large MTUs that have no
choice but to drop the traffic and hope for a retransmit. That's why they
have the highatomic reserve. If the reserve is used for normal unmovable
allocations then allocation requests that could have waited for reclaim
may cause high-order atomic allocations to fail. Changing it may allow
improve latencies in some limited cases while causing functional failures
in others.  If there is a special case where there are a large number of
other high-order allocations then I would suggest increasing min_free_kbytes
instead as a workaround.

-- 
Mel Gorman
SUSE Labs