On Mon, Jul 25, 2016 at 4:47 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > Can this suffering be quantified please? > The observed suffering is primarily visible within an IBM Qradar installation. From a high level, the lower limit to the amount of advisory readahead pages results in a 3-5x increase in time necessary to complete an identical query within the application. Note, all of the below values are with Readahead configured to 64Kib. Baseline behaviour - Prior to: 600e19af ("mm: use only per-device readahead limit") 6d2be915 ("mm/readahead.c: fix readahead failure for memoryless NUMA nodes and limit readahead pages") Result: Qradar - Command: "username equals root" - 57.3s to complete search New performance - With: 600e19af ("mm: use only per-device readahead limit") 6d2be915 ("mm/readahead.c: fix readahead failure for memoryless NUMA nodes and limit readahead pages") Result: Qradar - "username equals root" query - 245.7s to complete search Proposed behaviour - With the proposed patch in place. Result: Qradar - "username equals root" query - 57s to complete search In narrowing the source of the performance deficit, it was observed that the amount of data loaded into pagecache via madvise was quite a bit lower following the noted commits. As simply reverting those lower limits were not accepted previously, the proposed alternative strategy seemed like the most beneficial path forwards. > > Linus probably has opinions ;) > I understand that changes to readahead that are very similar have been proposed quite a bit lately. If there are any changes or testing needed, I'm more than happy to tackle that. Thank you in advance! -- Kyle Walker -- 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>