On Mon, Feb 19, 2018 at 12:26:39PM +0000, Robert Harris wrote: > > > > On 19 Feb 2018, at 09:47, Mel Gorman <mgor...@suse.de> wrote: > > > > On Sun, Feb 18, 2018 at 04:47:55PM +0000, robert.m.har...@oracle.com wrote: > >> From: "Robert M. Harris" <robert.m.har...@oracle.com> > >> > >> __fragmentation_index() calculates a value used to determine whether > >> compaction should be favoured over page reclaim in the event of allocation > >> failure. The calculation itself is opaque and, on inspection, does not > >> match its existing description. The function purports to return a value > >> between 0 and 1000, representing units of 1/1000. Barring the case of a > >> pathological shortfall of memory, the lower bound is instead 500. This is > >> significant because it is the default value of sysctl_extfrag_threshold, > >> i.e. the value below which compaction should be avoided in favour of page > >> reclaim for costly pages. > >> > >> This patch implements and documents a modified version of the original > >> expression that returns a value in the range 0 <= index < 1000. It amends > >> the default value of sysctl_extfrag_threshold to preserve the existing > >> behaviour. > >> > >> Signed-off-by: Robert M. Harris <robert.m.har...@oracle.com> > > > > You have to update sysctl_extfrag_threshold as well for the new bounds. > > This patch makes its default value zero. >
Sorry, I'm clearly blind. > > It effectively makes it a no-op but it was a no-op already and adjusting > > that default should be supported by data indicating it's safe. > > Would it be acceptable to demonstrate using tracing that in both the > pre- and post-patch cases > > 1. compaction is attempted regardless of fragmentation index, > excepting that > > 2. reclaim is preferred even for non-zero fragmentation during > an extreme shortage of memory > If you can demonstrate that for both reclaim-intensive and compaction-intensive workloads then yes. Also include the reclaim and compaction stats from /proc/vmstat and not just tracepoints to demonstrate that reclaim doesn't get out of control and reclaim the world in response to failed high-order allocations such as THP. -- Mel Gorman SUSE Labs
