On Thu 03-10-19 12:52:33, David Rientjes wrote: > On Thu, 3 Oct 2019, Vlastimil Babka wrote: > > > I think the key differences between Mike's tests and Michal's is this part > > from Mike's mail linked above: > > > > "I 'tested' by simply creating some background activity and then seeing > > how many hugetlb pages could be allocated. Of course, many tries over > > time in a loop." > > > > - "some background activity" might be different than Michal's pre-filling > > of the memory with (clean) page cache > > - "many tries over time in a loop" could mean that kswapd has time to > > reclaim and eventually the new condition for pageblock order will pass > > every few retries, because there's enough memory for compaction and it > > won't return COMPACT_SKIPPED > > > > I'll rely on Mike, the hugetlb maintainer, to assess the trade-off between > the potential for encountering very expensive reclaim as Andrea did and > the possibility of being able to allocate additional hugetlb pages at > runtime if we did that expensive reclaim. That tradeoff has been expressed by __GFP_RETRY_MAYFAIL which got broken by b39d0ee2632d. > For parity with previous kernels it seems reasonable to ask that this > remains unchanged since allocating large amounts of hugetlb pages has > different latency expectations than during page fault. This patch is > available if he'd prefer to go that route. > > On the other hand, userspace could achieve similar results if it were to > use vm.drop_caches and explicitly triggered compaction through either > procfs or sysfs before writing to vm.nr_hugepages, and that would be much > faster because it would be done in one go. Users who allocate through the > kernel command line would obviously be unaffected. Requesting the userspace to drop _all_ page cache in order allocate a number of hugetlb pages or any other affected __GFP_RETRY_MAYFAIL requests is simply not reasonable IMHO. -- Michal Hocko SUSE Labs
