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
http://oracle-base.com/articles/linux/configuring-huge-pages-for-oracle-on-linux-64.php

Thanks,
Davidlohr

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>