On 02/24/2015 11:32 AM, Vlastimil Babka wrote: > On 02/23/2015 11:56 PM, Andrew Morton wrote: >> On Mon, 23 Feb 2015 14:46:43 -0800 Davidlohr Bueso <d...@stgolabs.net> wrote: >> >>> On Mon, 2015-02-23 at 13:58 +0100, Vlastimil Babka wrote: >>>> Recently, there was concern expressed (e.g. [1]) whether the quite >>>> aggressive >>>> THP allocation attempts on page faults are a good performance trade-off. >>>> >>>> - THP allocations add to page fault latency, as high-order allocations are >>>> notoriously expensive. Page allocation slowpath now does extra checks >>>> for >>>> GFP_TRANSHUGE && !PF_KTHREAD to avoid the more expensive synchronous >>>> compaction for user page faults. But even async compaction can be >>>> expensive. >>>> - During the first page fault in a 2MB range we cannot predict how much of >>>> the >>>> range will be actually accessed - we can theoretically waste as much as >>>> 511 >>>> worth of pages [2]. Or, the pages in the range might be accessed from >>>> CPUs >>>> from different NUMA nodes and while base pages could be all local, THP >>>> could >>>> be remote to all but one CPU. The cost of remote accesses due to this >>>> false >>>> sharing would be higher than any savings on the TLB. >>>> - The interaction with memcg are also problematic [1]. >>>> >>>> Now I don't have any hard data to show how big these problems are, and I >>>> expect we will discuss this on LSF/MM (and hope somebody has such data >>>> [3]). >>>> But it's certain that e.g. SAP recommends to disable THPs [4] for their >>>> apps >>>> for performance reasons. >>> >>> There are plenty of examples of this, ie for Oracle: >>> >>> https://blogs.oracle.com/linux/entry/performance_issues_with_transparent_huge >> >> hm, five months ago and I don't recall seeing any followup to this. > > Actually it's year + five months, but nevertheless... > >> Does anyone know what's happening?
So I think that post was actually about THP support enabled in .config slowing down hugetlbfs, and found a followup post here https://blogs.oracle.com/linuxkernel/entry/performance_impact_of_transparent_huge and that was after all solved in 3.12. Sasha also mentioned that split PTL patchset helped as well, and the degradation in IOPS due to THP enabled is now limited to 5%, and possibly the refcounting redesign could help. That however means the workload is based on hugetlbfs and shouldn't trigger THP page fault activity, which is the aim of this patchset. Some more googling made me recall that last LSF/MM, postgresql people mentioned THP issues and pointed at compaction. See http://lwn.net/Articles/591723/ That's exactly where this patchset should help, but I obviously won't be able to measure this before LSF/MM... I'm CCing the psql guys from last year LSF/MM - do you have any insight about psql performance with THPs enabled/disabled on recent kernels, where e.g. compaction is no longer synchronous for THP page faults? Thanks, Vlastimil -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
