On Tue, Apr 06, 2010 at 05:06:13PM -0700, Andrew Morton wrote: > On Fri, 2 Apr 2010 17:02:46 +0100 > Mel Gorman <mel@xxxxxxxxx> wrote: > > > The kernel applies some heuristics when deciding if memory should be > > compacted or reclaimed to satisfy a high-order allocation. One of these > > is based on the fragmentation. If the index is below 500, memory will > > not be compacted. This choice is arbitrary and not based on data. To > > help optimise the system and set a sensible default for this value, this > > patch adds a sysctl extfrag_threshold. The kernel will only compact > > memory if the fragmentation index is above the extfrag_threshold. > > Was this the most robust, reliable, no-2am-phone-calls thing we could > have done? > > What about, say, just doing a bit of both until something worked? I guess you could but that is not a million miles away from what currently happens. This heuristic is basically "based on free memory layout, how likely is compaction to succeed?". It makes a decision based on that. A later patch then checks if the guess was right. If not, just try direct reclaim for a bit before trying compaction again. > For > extra smarts we could remember what worked best last time, and make > ourselves more likely to try that next time. > With the later patch, this is essentially what we do. Granted we remember the opposite "If the kernel guesses wrong, then don't compact for a short while before trying again". > Or whatever, but extfrag_threshold must die! And replacing it with a > hardwired constant doesn't count ;) > I think what you have in mind is "just try compaction every time" but my concern about that is we'll hit a corner case where a lot of CPU time is taken scanning zones uselessly. That is what this heuristic and the back-off logic in a later patch was meant to avoid. I haven't thought of a better alternative :/ -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>