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

[Mel Gorman]

On Tue, Sep 26, 2017 at 06:04:04PM +0800, Hui Zhu wrote:
> 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.
> 

That can already happen through the migratetype fallback lists.

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

Technically, yes but the barrier to entry will be high as you'll have to
explain carefully why it is necessary including information on why order-0
pages cannot be used, back it up with data showing what is improved as a
result and justify why potentially forcing normal workloads to reclaim due
to being unable to use the high-order reserve is ok. If it's a limitation
of a specific driver then it'll be asked why that driver does not have a
dedicated pool (which is functionally similar to having a dedicated reserve).

-- 
Mel Gorman
SUSE Labs