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

[Hui Zhu]

2017-09-26 17:51 GMT+08:00 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>:
> 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.

I think let 0 order unmovable page alloc and other order unmovable pages
alloc use different migrate types will help against long-term
fragmentation.

Do you think kernel can add a special migrate type for big than 0 order
unmovable pages alloc?

Thanks,
Hui

>
> --
> Mel Gorman
> SUSE Labs