On Fri, Feb 10, 2017 at 06:23:36PM +0100, Vlastimil Babka wrote: > The __rmqueue_fallback() function is called when there's no free page of > requested migratetype, and we need to steal from a different one. There are > various heuristics to make this event infrequent and reduce permanent > fragmentation. The main one is to try stealing from a pageblock that has the > most free pages, and possibly steal them all at once and convert the whole > pageblock. Precise searching for such pageblock would be expensive, so instead > the heuristics walks the free lists from MAX_ORDER down to requested order and > assumes that the block with highest-order free page is likely to also have the > most free pages in total. > > Chances are that together with the highest-order page, we steal also pages of > lower orders from the same block. But then we still split the highest order > page. This is wasteful and can contribute to fragmentation instead of avoiding > it. >
The original intent was that if an allocation request was stealing a pageblock that taking the largest one would reduce the likelihood of a steal in the near future by the same type. > This patch thus changes __rmqueue_fallback() to just steal the page(s) and put > them on the freelist of the requested migratetype, and only report whether it > was successful. Then we pick (and eventually split) the smallest page with > __rmqueue_smallest(). This all happens under zone lock, so nobody can steal > it > from us in the process. This should reduce fragmentation due to fallbacks. At > worst we are only stealing a single highest-order page and waste some cycles > by > moving it between lists and then removing it, but fallback is not exactly hot > path so that should not be a concern. As a side benefit the patch removes some > duplicate code by reusing __rmqueue_smallest(). > > Signed-off-by: Vlastimil Babka <[email protected]> But conceptually this is better so Acked-by: Mel Gorman <[email protected]> -- Mel Gorman SUSE Labs
