On Mon, 23 Feb 2015 14:46:43 -0800 Davidlohr Bueso <[email protected]> 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. Does anyone know what's happening? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
