On 03/16/2017 02:30 AM, Joonsoo Kim wrote: > Hello, Hi, sorry for the late replies. > On Tue, Mar 07, 2017 at 02:15:39PM +0100, Vlastimil Babka wrote: >> When detecting whether compaction has succeeded in forming a high-order page, >> __compact_finished() employs a watermark check, followed by an own search for >> a suitable page in the freelists. This is not ideal for two reasons: >> >> - The watermark check also searches high-order freelists, but has a less strict >> criteria wrt fallback. It's therefore redundant and waste of cycles. This was >> different in the past when high-order watermark check attempted to apply >> reserves to high-order pages. > > Although it looks redundant now, I don't like removal of the watermark > check here. Criteria in watermark check would be changed to more strict > later and we would easily miss to apply it on compaction side if the > watermark check is removed. I see, but compaction is already full of various watermark(-like) checks that have to be considered/updated if watermark checking changes significantly, or things will go subtly wrong. I doubt this extra check can really help much in such cases. >> >> - The watermark check might actually fail due to lack of order-0 pages. >> Compaction can't help with that, so there's no point in continuing because of >> that. It's possible that high-order page still exists and it terminates. > > If lack of order-0 pages is the reason for stopping compaction, we > need to insert the watermark check for order-0 to break the compaction > instead of removing it. Am I missing something? You proposed that once IIRC, but didn't follow up? Currently we learn about insufficient order-0 watermark in __isolate_free_page() from the free scanner. We could potentially stop compacting earlier by checking it also in compact_finished(), but maybe it doesn't happen that often and it's just extra checking overhead. So I wouldn't be terribly opposed by converting the current check to an order-0 fail-compaction check (instead of removing it), but I really wouldn't like to insert the order-0 one and also keep the current one. > Thanks. > -- 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>
