linux-kernel - Re: [PATCH 0/6] PM / Hibernate: Memory bitmap scalability improvements

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Message-ID: <2105930.LcBFYOUaLE@vostro.rjw.lan>
Date:	Sat, 19 Jul 2014 00:05:02 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] PM / Hibernate: Memory bitmap scalability improvements

On Friday, July 18, 2014 01:57:17 PM Joerg Roedel wrote:
> Hi,
> 
> here is a patch set to improve the scalability of the memory
> bitmap implementation used for hibernation. The current
> implementation does not scale well to machines with several
> TB of memory. A resume on those machines may cause soft
> lockups to be reported.
> 
> These patches improve the data structure by adding a radix
> tree to the linked list structure to improve random access
> performance from O(n) to O(log_b(n)), where b depends on the
> architecture (b=512 on amd64, 1024 in i386).
> 
> A test on a 12TB machine showed an improvement in resume
> time from 76s with the old implementation to 2.4s with the
> radix tree and the improved swsusp_free function. See below
> for details of this test.
> 
> Patches 1-3 that add the radix tree while keeping the
> existing memory bitmap implementation in place and add code
> to compare the results between both implementations. This
> was used during development to make sure both data
> structures return the same results.
> 
> Patch 4 re-implements the swsusp_free() function to not
> iterate over all pfns but only over the bits set in the
> bitmaps. This showed to scale better on large memory
> machines.
> 
> Patch 5 removes the old memory bitmap implementation now
> that the radix tree is in place and working correctly.
> 
> The last patch adds touching the soft lockup watchdog in
> rtree_next_node. This is necessary because the worst case
> performance (all bits set in the forbidden_pages_map and
> free_pages_map) is the same as with the old implementation
> and may still cause soft lockups. Patch 6 avoids this.
> 
> The code was tested in 32 and 64 bit x86 and showed no
> issues there.
> 
> Below is an example test that shows the performance
> improvement on a 12TB machine. First the test with the old
> memory bitmap:
> 
> # time perf record /usr/sbin/resume $sdev
> resume: libgcrypt version: 1.5.0
> [ perf record: Woken up 12 times to write data ]
> [ perf record: Captured and wrote 2.882 MB perf.data (~125898 samples) ]
> 
> real    1m16.043s
> user    0m0.016s
> sys     0m0.312s
> # perf report --stdio |head -50
> # Events: 75K cycles
> #
> # Overhead  Command         Shared Object                                   
> Symbol
> # ........  .......  .................... 
> ........................................
> #
>     56.16%   resume  [kernel.kallsyms]     [k] memory_bm_test_bit
>     19.35%   resume  [kernel.kallsyms]     [k] swsusp_free
>     14.90%   resume  [kernel.kallsyms]     [k] memory_bm_find_bit
>      7.28%   resume  [kernel.kallsyms]     [k] swsusp_page_is_forbidden
> 
> And here is the same test on the same machine with these
> patches applied:
> 
> #  time perf record /usr/sbin/resume $sdev
> resume: libgcrypt version: 1.5.0
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.039 MB perf.data (~1716 samples) ]
> 
> real    0m2.376s
> user    0m0.020s
> sys     0m0.408s
> 
> # perf report --stdio |head -50
> # Events: 762  cycles
> #
> # Overhead  Command      Shared Object                     Symbol
> # ........  .......  .................  .........................
> #
>     34.78%   resume  [kernel.kallsyms]  [k] find_next_bit
>     27.03%   resume  [kernel.kallsyms]  [k] clear_page_c_e
>      9.70%   resume  [kernel.kallsyms]  [k] mark_nosave_pages
>      3.92%   resume  [kernel.kallsyms]  [k] alloc_rtree_node
>      2.38%   resume  [kernel.kallsyms]  [k] get_image_page
> 
> As can be seen on these results these patches improve the
> scalability significantly. Please review, any comments
> appreciated.

Looks good.

How much testing did that receive?

Rafael

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