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From: "Huang\, Ying" <>
To: Johannes Weiner <>
Cc: "Huang\, Ying" <>,
	Andrew Morton <>, <>,
Subject: Re: [PATCH -mm -v9 2/3] mm, THP, swap: Check whether THP can be split firstly
Date: Thu, 20 Apr 2017 08:50:43 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Johannes Weiner's message of "Wed, 19 Apr 2017 12:13:18 -0400")

Johannes Weiner <> writes:

> On Wed, Apr 19, 2017 at 03:06:24PM +0800, Huang, Ying wrote:
>> From: Huang Ying <>
>> To swap out THP (Transparent Huage Page), before splitting the THP,
>> the swap cluster will be allocated and the THP will be added into the
>> swap cache.  But it is possible that the THP cannot be split, so that
>> we must delete the THP from the swap cache and free the swap cluster.
>> To avoid that, in this patch, whether the THP can be split is checked
>> firstly.  The check can only be done racy, but it is good enough for
>> most cases.
>> With the patchset, the swap out throughput improves 3.6% (from about
>> 4.16GB/s to about 4.31GB/s) in the vm-scalability swap-w-seq test case
>> with 8 processes.  The test is done on a Xeon E5 v3 system.  The swap
>> device used is a RAM simulated PMEM (persistent memory) device.  To
>> test the sequential swapping out, the test case creates 8 processes,
>> which sequentially allocate and write to the anonymous pages until the
>> RAM and part of the swap device is used up.
>> Cc: Johannes Weiner <>
>> Signed-off-by: "Huang, Ying" <>
>> Acked-by: Kirill A. Shutemov <> [for can_split_huge_page()]
> How often does this actually happen in practice? Because all that this
> protects us from is trying to allocate a swap cluster - which with the
> si->free_clusters list really isn't all that expensive - and return it
> again. Unless this happens all the time in practice, this optimization
> seems misplaced.

In addition to allocate/free swap cluster, add/delete to/from the swap
cache will be called too.

> It's especially a little strange because in the other email I asked
> about the need for unlikely() annotations, yet this patch is adding
> branches and checks for what seems to be an unlikely condition into
> the THP hot path.
> I'd suggest you drop both these optimization attempts unless there is
> real data proving that they have a measurable impact.

To my surprise too, I found this patch has measurable impact in my
test.  The swap out throughput improves 3.6% in the vm-scalability
swap-w-seq test case with 8 processes.  Details are in the original
patch description.

Best Regards,
Huang, Ying

  reply	other threads:[~2017-04-20  0:50 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-19  7:06 [PATCH -mm -v9 0/3] THP swap: Delay splitting THP during swapping out Huang, Ying
2017-04-19  7:06 ` [PATCH -mm -v9 1/3] mm, THP, swap: Delay splitting THP during swap out Huang, Ying
2017-04-19 15:52   ` Johannes Weiner
2017-04-20  0:43     ` Huang, Ying
2017-04-21  6:11   ` Balbir Singh
2017-04-21  7:21     ` Huang, Ying
2017-04-21 14:08     ` Johannes Weiner
2017-04-22  0:07       ` Huang, Ying
2017-04-19  7:06 ` [PATCH -mm -v9 2/3] mm, THP, swap: Check whether THP can be split firstly Huang, Ying
2017-04-19 16:13   ` Johannes Weiner
2017-04-20  0:50     ` Huang, Ying [this message]
2017-04-20 20:50       ` Johannes Weiner
2017-04-21  0:34         ` Huang, Ying
2017-04-19  7:06 ` [PATCH -mm -v9 3/3] mm, THP, swap: Enable THP swap optimization only if has compound map Huang, Ying
2017-04-19 16:00   ` Johannes Weiner
2017-04-20  0:51     ` Huang, Ying

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