On Tue, Mar 23, 2010 at 02:27:08PM -0500, Christoph Lameter wrote: > On Tue, 23 Mar 2010, Mel Gorman wrote: > > > I was having some sort of fit when I wrote that obviously. Try this on > > for size > > > > The fragmentation index may indicate that a failure is due to external > > fragmentation but after a compaction run completes, it is still possible > > for an allocation to fail. > > Ok. > > > > > fail. There are two obvious reasons as to why > > > > > > > > o Page migration cannot move all pages so fragmentation remains > > > > o A suitable page may exist but watermarks are not met > > > > > > > > In the event of compaction and allocation failure, this patch prevents > > > > compaction happening for a short interval. It's only recorded on the > > > > > > compaction is "recorded"? deferred? > > > > > > > deferred makes more sense. > > > > What I was thinking at the time was that compact_resume was stored in struct > > zone - i.e. that is where it is recorded. > > Ok adding a dozen or more words here may be useful. > In the event of compaction followed by an allocation failure, this patch defers further compaction in the zone for a period of time. The zone that is deferred is the first zone in the zonelist - i.e. the preferred zone. To defer compaction in the other zones, the information would need to be stored in the zonelist or implemented similar to the zonelist_cache. This would impact the fast-paths and is not justified at this time. ? > > > > preferred zone but that should be enough coverage. This could have been > > > > implemented similar to the zonelist_cache but the increased size of the > > > > zonelist did not appear to be justified. > > > > > > > @@ -1787,6 +1787,9 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order, > > > > */ > > > > count_vm_event(COMPACTFAIL); > > > > > > > > + /* On failure, avoid compaction for a short time. */ > > > > + defer_compaction(preferred_zone, jiffies + HZ/50); > > > > + > > > > > > 20ms? How was that interval determined? > > > > > > > Matches the time the page allocator would defer to an event like > > congestion. The choice is somewhat arbitrary. Ideally, there would be > > some sort of event that would re-enable compaction but there wasn't an > > obvious candidate so I used time. > > There are frequent uses of HZ/10 as well especially in vmscna.c. A longer > time may be better? HZ/50 looks like an interval for writeout. But this > is related to reclaim? > HZ/10 is somewhat of an arbitrary choice as well and there isn't data on which is better and which is worse. If the zone is full of dirty data, then HZ/10 makes sense for IO. If it happened to be mainly clean cache but under heavy memory pressure, then reclaim would be a relatively fast event and a shorter wait makes sense of HZ/50. Thing is, if we start with a short timer and it's too short, COMPACTFAIL will be growing steadily. If we choose a long time and it's too long, there is no counter to indicate it was a bad choice. Hence, I'd prefer the short timer to start with and ideally resume compaction after some event in the future rather than depending on time. Does that make sense? > > backing-dev.h <global> 283 long congestion_wait(int sync, long timeout); > 1 backing-dev.c <global> 762 EXPORT_SYMBOL(congestion_wait); > 2 usercopy_32.c __copy_to_user_ll 754 congestion_wait(BLK_RW_ASYNC, HZ/50); > 3 pktcdvd.c pkt_make_request 2557 congestion_wait(BLK_RW_ASYNC, HZ); > 4 dm-crypt.c kcryptd_crypt_write_convert 834 congestion_wait(BLK_RW_ASYNC, HZ/100); > 5 file.c fat_file_release 137 congestion_wait(BLK_RW_ASYNC, HZ/10); > 6 journal.c reiserfs_async_progress_wait 990 congestion_wait(BLK_RW_ASYNC, HZ / 10); > 7 kmem.c kmem_alloc 61 congestion_wait(BLK_RW_ASYNC, HZ/50); > 8 kmem.c kmem_zone_alloc 117 congestion_wait(BLK_RW_ASYNC, HZ/50); > 9 xfs_buf.c _xfs_buf_lookup_pages 343 congestion_wait(BLK_RW_ASYNC, HZ/50); > a backing-dev.c congestion_wait 751 long congestion_wait(int sync, long timeout) > b memcontrol.c mem_cgroup_force_empty 2858 congestion_wait(BLK_RW_ASYNC, HZ/10); > c page-writeback.c throttle_vm_writeout 674 congestion_wait(BLK_RW_ASYNC, HZ/10); > d page_alloc.c __alloc_pages_high_priority 1753 congestion_wait(BLK_RW_ASYNC, HZ/50); > e page_alloc.c __alloc_pages_slowpath 1924 congestion_wait(BLK_RW_ASYNC, HZ/50); > f vmscan.c shrink_inactive_list 1136 congestion_wait(BLK_RW_ASYNC, HZ/10); > g vmscan.c shrink_inactive_list 1220 congestion_wait(BLK_RW_ASYNC, HZ/10); > h vmscan.c do_try_to_free_pages 1837 congestion_wait(BLK_RW_ASYNC, HZ/10); > i vmscan.c balance_pgdat 2161 congestion_wait(BLK_RW_ASYNC, HZ/10); > -- 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>