From: "Huang\, Ying" <ying.huang@intel.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Huang\, Ying" <ying.huang@intel.com>,
Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
<linux-kernel@vger.kernel.org>
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: <87efwnrjfg.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <20170419161318.GC3376@cmpxchg.org> (Johannes Weiner's message of "Wed, 19 Apr 2017 12:13:18 -0400")
Johannes Weiner <hannes@cmpxchg.org> writes:
> On Wed, Apr 19, 2017 at 03:06:24PM +0800, Huang, Ying wrote:
>> From: Huang Ying <ying.huang@intel.com>
>>
>> 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 <hannes@cmpxchg.org>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> [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
next prev parent 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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