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SubjectRe: [RFC v2][PATCH]page_fault retry with NOPAGE_RETRY
On 2008-12-05 21:40, Ying Han wrote:
> changelog[v2]:
> - reduce the runtime overhead by extending the 'write' flag of
> handle_mm_fault() to indicate the retry hint.
> - add another two branches in filemap_fault with retry logic.
> - replace find_lock_page with find_lock_page_retry to make the code
> cleaner.
>
> todo:
> - there is potential a starvation hole with the retry. By the time the
> retry returns, the pages might be released. we can make change by holding
> page reference as well as remembering what the page "was"(in case the
> file was truncated). any suggestion here are welcomed.
>
> I also made patches for all other arch. I am posting x86_64 here first and
> i will post others by the time everyone feels comfortable of this patch.
>
> Edwin, please test this patch with your testcase and check if you get any
> performance improvement of mmap over read. I added another two more places
> in filemap_fault with retry logic which you might hit in your privous
> experiment.
>

I get much better results with this patch than with v1, thanks!

mmap now scales almost as well as read does (there is a small ~5%
overhead), which is a significant improvement over not scaling at all!

Here are the results when running my testcase:

Number of threads ->, 1,,, 2,,, 4,,, 8,,, 16
Kernel version, read, mmap, mixed, read, mmap, mixed, read, mmap, mixed,
read, mmap, mixed, read, mmap, mixed
2.6.28-rc7-tip, 27.55, 26.18, 27.06, 16.18, 16.97, 16.10, 11.06, 11.64,
11.41, 9.38, 9.97, 9.31, 9.37, 9.82, 9.3


Here are the /proc/lock_stat output when running my testcase, contention
is lower (34911+10462 vs 58590+7231), and waittime-total is better
(57 601 464 vs 234 170 024)

lock_stat version 0.3
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions
waittime-min waittime-max waittime-total acq-bounces
acquisitions holdtime-min holdtime-max holdtime-total
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&mm->mmap_sem-W: 5843
10462 2.89 138824.72 14217159.52
18965 84205 1.81 5031.07 725293.65
&mm->mmap_sem-R: 20208
34911 4.87 136797.26 57601464.49 55797
1110394 1.89 164918.52 30551371.71
---------------
&mm->mmap_sem 5341
[<ffffffff802bf9d7>] sys_munmap+0x47/0x80
&mm->mmap_sem 28579
[<ffffffff805d1c62>] do_page_fault+0x172/0xab0
&mm->mmap_sem 5030
[<ffffffff80211161>] sys_mmap+0xf1/0x140
&mm->mmap_sem 6331
[<ffffffff802a675e>] find_lock_page_retry+0xde/0xf0
---------------
&mm->mmap_sem 13558
[<ffffffff802a675e>] find_lock_page_retry+0xde/0xf0
&mm->mmap_sem 4694
[<ffffffff802bf9d7>] sys_munmap+0x47/0x80
&mm->mmap_sem 3681
[<ffffffff80211161>] sys_mmap+0xf1/0x140
&mm->mmap_sem 23374
[<ffffffff805d1c62>] do_page_fault+0x172/0xab0

On clamd:

Here holdtime-total is better (1 493 154 + 2 395 987 vs 2 087 538 + 2
514 673), and number of contentions on read
(458 052 vs 5851


lock_stat version 0.3
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
class name con-bounces contentions
waittime-min waittime-max waittime-total acq-bounces
acquisitions holdtime-min holdtime-max holdtime-total
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
&mm->mmap_sem-W: 346769
533541 1.62 99819.40 454843342.63
411259 588486 1.33 6719.62 2395987.75
&mm->mmap_sem-R: 197856
458052 1.59 99800.28 313508721.01
338158 653427 1.71 25421.10 1493154.95
---------------
&mm->mmap_sem 427857
[<ffffffff805d1c62>] do_page_fault+0x172/0xab0
&mm->mmap_sem 266464
[<ffffffff802bf9d7>] sys_munmap+0x47/0x80
&mm->mmap_sem 251689
[<ffffffff802110d6>] sys_mmap+0x66/0x140
&mm->mmap_sem 15187
[<ffffffff80211161>] sys_mmap+0xf1/0x140
---------------
&mm->mmap_sem 226908
[<ffffffff802110d6>] sys_mmap+0x66/0x140
&mm->mmap_sem 483909
[<ffffffff805d1c62>] do_page_fault+0x172/0xab0
&mm->mmap_sem 229404
[<ffffffff802bf9d7>] sys_munmap+0x47/0x80
&mm->mmap_sem 13229
[<ffffffff80211161>] sys_mmap+0xf1/0x140
...............................................................................................................................................................................................
&sem->wait_lock: 112617
114394 0.41 111.20 225590.14
1517470 6300681 0.27 4103.77 3814684.55
---------------
&sem->wait_lock 5634
[<ffffffff8043a608>] __up_write+0x28/0x170
&sem->wait_lock 13595
[<ffffffff805ce4dc>] __down_read+0x1c/0xbc
&sem->wait_lock 38882
[<ffffffff8043a4a0>] __down_read_trylock+0x20/0x60
&sem->wait_lock 30718
[<ffffffff8043a773>] __up_read+0x23/0xc0
---------------
&sem->wait_lock 21389
[<ffffffff8043a4a0>] __down_read_trylock+0x20/0x60
&sem->wait_lock 48959
[<ffffffff8043a608>] __up_write+0x28/0x170
&sem->wait_lock 24330
[<ffffffff8043a773>] __up_read+0x23/0xc0
&sem->wait_lock 9000
[<ffffffff805ce4dc>] __down_read+0x1c/0xbc

> @@ -694,6 +694,7 @@ static inline int page_mapped(struct page *page)
> #define VM_FAULT_SIGBUS 0x0002
> #define VM_FAULT_MAJOR 0x0004
> #define VM_FAULT_WRITE 0x0008 /* Special case for get_user_pages */
> +#define VM_FAULT_RETRY 0x0010
>
> #define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page
>

The patch got damaged here, and failed to apply, I added the missing */,
and then git-am -3 applied it.

Best regards,
--Edwin


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