On Mon, Jun 22, 2020 at 06:40:20PM +0900, ????????? wrote: > >But more importantly, I have hard time to follow why we need both > >zone_watermark_fast and zone_watermark_ok now. They should be > >essentially the same for anything but order == 0. For order 0 the > >only difference between the two is that zone_watermark_ok checks for > >ALLOC_HIGH resp ALLOC_HARDER, ALLOC_OOM. So what is exactly fast about > >the former and why do we need it these days? > > > > I think the author, Mel, may ansewr. But I think the wmark_fast may > fast by 1) not checking more condition about wmark and 2) using inline > rather than function. According to description on commit 48ee5f3696f6, > it seems to bring about 4% improvement. > The original intent was that watermark checks were expensive as some of the calculations are only necessary when a zone is relatively low on memory and the check does not always have to be 100% accurate. This is probably still true given that __zone_watermark_ok() makes a number of calculations depending on alloc flags even if a zone is almost completely free. -- Mel Gorman SUSE Labs
