On Fri, 27 Feb 2015, Rik van Riel 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. > I would say it specifies how much unmapped memory can become mapped by a hugepage. > 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... > I know that the value of 64 would also be undesirable for Google since we tightly constrain memory usage, we have used max_ptes_none == 0 since it was introduced. We can get away with that because our malloc() is modified to try to give back large contiguous ranges of memory periodically back to the system, also using madvise(MADV_DONTNEED), and tries to avoid splitting thp memory. The value is determined by how the system will be used: do you tightly constrain memory usage and not allow any unmapped memory be collapsed into a hugepage, or do you have an abundance of memory and really want an aggressive value like HPAGE_PMD_NR-1. Depending on the properties of the system, you can tune this to anything you want just like we do in initscripts. I'm only concerned here about changing a default that has been around for four years and the possibly negative implications that will have on users who never touch this value. They undoubtedly get less memory backed by thp, and that can lead to a performance regression. So if this patch is merged and we get a bug report for the 4.1 kernel, do we tell that user that we changed behavior out from under them and to adjust the tunable back to HPAGE_PMD_NR-1? Meanwhile, the bug report you cite has a workaround that has always been available for thp kernels: # echo 64 > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none -- 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>