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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
