Re: [RFC 0/6] the big khugepaged redesign

From: Andrew Morton
Date: Mon Feb 23 2015 - 17:56:25 EST

On Mon, 23 Feb 2015 14:46:43 -0800 Davidlohr Bueso <dave@xxxxxxxxxxxx> 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:

hm, five months ago and I don't recall seeing any followup to this.
Does anyone know what's happening?

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