Re: [RFC 0/6] the big khugepaged redesign
From: Davidlohr Bueso
Date: Mon Feb 23 2015 - 17:47:07 EST
On Mon, 2015-02-23 at 13:58 +0100, Vlastimil Babka wrote:
> Recently, there was concern expressed (e.g. ) 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 . 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 .
> 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 ).
> But it's certain that e.g. SAP recommends to disable THPs  for their apps
> for performance reasons.
There are plenty of examples of this, ie for Oracle:
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