On 02/27/2015 03:53 PM, David Rientjes wrote: > On Fri, 27 Feb 2015, Ebru Akagunduz wrote: > >> Using THP, programs can access memory faster, by having the >> kernel collapse small pages into large pages. The parameter >> max_ptes_none specifies how many extra small pages (that are >> not already mapped) can be allocated when collapsing a group >> of small pages into one large page. >> > > Not exactly, khugepaged isn't "allocating" small pages to collapse into a > hugepage, rather it is allocating a hugepage and then remapping the > pageblock's mapped pages. How would you describe the amount of extra memory allocated, as a result of converting a partially mapped 2MB area into a THP? It is not physically allocating 4kB pages, but I would like to keep the text understandable to people who do not know the THP internals. >> A larger value of max_ptes_none can cause the kernel >> to collapse more incomplete areas into THPs, speeding >> up memory access at the cost of increased memory use. >> A smaller value of max_ptes_none will reduce memory >> waste, at the expense of collapsing fewer areas into >> THPs. >> > > This changelog only describes what max_ptes_none does, it doesn't state > why you want to change it from HPAGE_PMD_NR-1, which is 511 on x86_64 > (largest value, more thp), to HPAGE_PMD_NR/8, which is 64 (smaller value, > less thp, less rss as a result of collapsing). > > This has particular performance implications on users who already have thp > enabled, so it's difficult to change the default. This is tuanble that > you could easily set in an initscript, so I don't think we need to change > the value for everybody. I think we do need to change the default. Why? See this bug: >> The problem was reported here: >> https://bugzilla.kernel.org/show_bug.cgi?id=93111 Now, there may be a better value than HPAGE_PMD_NR/8, but I am not sure what it would be, or why. I do know that HPAGE_PMD_NR-1 results in undesired behaviour, as seen in the bug above... -- All rights reversed -- 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>