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* [PATCH v4 0/4] make vm_committed_as_batch aware of vm overcommit policy
@ 2020-05-29  1:06 Feng Tang
  2020-05-29  1:06 ` [PATCH v4 1/4] proc/meminfo: avoid open coded reading of vm_committed_as Feng Tang
                   ` (3 more replies)
  0 siblings, 4 replies; 16+ messages in thread
From: Feng Tang @ 2020-05-29  1:06 UTC (permalink / raw)
  To: Andrew Morton, Michal Hocko, Johannes Weiner, Matthew Wilcox,
	Mel Gorman, Kees Cook, Qian Cai, andi.kleen, tim.c.chen,
	dave.hansen, ying.huang, linux-mm, linux-kernel
  Cc: Feng Tang

When checking a performance change for will-it-scale scalability
mmap test [1], we found very high lock contention for spinlock of
percpu counter 'vm_committed_as':

    94.14%     0.35%  [kernel.kallsyms]         [k] _raw_spin_lock_irqsave
    48.21% _raw_spin_lock_irqsave;percpu_counter_add_batch;__vm_enough_memory;mmap_region;do_mmap;
    45.91% _raw_spin_lock_irqsave;percpu_counter_add_batch;__do_munmap;

Actually this heavy lock contention is not always necessary. The
'vm_committed_as' needs to be very precise when the strict
OVERCOMMIT_NEVER policy is set, which requires a rather small batch
number for the percpu counter.

So keep 'batch' number unchanged for strict OVERCOMMIT_NEVER policy,
and enlarge it for not-so-strict  OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS
policies.

Benchmark with the same testcase in [1] shows 53% improvement on a
8C/16T desktop, and 2097%(20X) on a 4S/72C/144T server. And for that
case, whether it shows improvements depends on if the test mmap size
is bigger than the batch number computed.

We tested 10+ platforms in 0day (server, desktop and laptop). If we
lift it to 64X, 80%+ platforms show improvements, and for 16X lift,
1/3 of the platforms will show improvements.

And generally it should help the mmap/unmap usage,as Michal Hocko
mentioned:
"
I believe that there are non-synthetic worklaods which would benefit
from a larger batch. E.g. large in memory databases which do large
mmaps during startups from multiple threads.
"

Note: There are some style complain from checkpatch for patch 3,
as sysctl handler declaration follows the similar format of sibling
functions

patch1: a cleanup for /proc/meminfo
patch2: a preparation patch which also improve the accuracy of
        vm_memory_committed
patch3: remove the VM_WARN_ONCE for vm_committed_as underflow check
patch4: main change

This is against today's linux-mm git tree on github.

Please help to review, thanks!

- Feng

----------------------------------------------------------------
Changelog:

  v4:
    * Remove the VM_WARN_ONCE check for vm_committed_as underflow,
      thanks to Qian Cai for finding and testing the warning

  v3:
    * refine commit log and cleanup code, according to comments
      from Michal Hocko and Matthew Wilcox
    * change the lift from 16X and 64X after test 
  
  v2:
     * add the sysctl handler to cover runtime overcommit policy
       change, as suggested by Andres Morton 
     * address the accuracy concern of vm_memory_committed()
       from Andi Kleen 


Feng Tang (4):
  proc/meminfo: avoid open coded reading of vm_committed_as
  mm/util.c: make vm_memory_committed() more accurate
  mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check
  mm: adjust vm_committed_as_batch according to vm overcommit policy

 fs/proc/meminfo.c    |  2 +-
 include/linux/mm.h   |  2 ++
 include/linux/mman.h |  4 ++++
 kernel/sysctl.c      |  2 +-
 mm/mm_init.c         | 18 ++++++++++++++----
 mm/util.c            | 22 +++++++++++++---------
 6 files changed, 35 insertions(+), 15 deletions(-)

-- 
2.7.4


^ permalink raw reply	[flat|nested] 16+ messages in thread
* Re: [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check
@ 2020-06-02  4:02 Qian Cai
  2020-06-03  9:48 ` Feng Tang
  0 siblings, 1 reply; 16+ messages in thread
From: Qian Cai @ 2020-06-02  4:02 UTC (permalink / raw)
  To: Feng Tang
  Cc: Andrew Morton, Michal Hocko, Johannes Weiner, Matthew Wilcox,
	Mel Gorman, Kees Cook, andi.kleen, tim.c.chen, dave.hansen,
	ying.huang, linux-mm, linux-kernel, Konstantin Khlebnikov



> On Jun 1, 2020, at 11:37 PM, Feng Tang <feng.tang@intel.com> wrote:
> 
> I re-run the same benchmark with v5.7 and 5.7+remove_warning kernels,
> the overall performance change is trivial (which is expected)
> 
>   1330147            +0.1%    1331032        will-it-scale.72.processes
> 
> But the perf stats of "self" shows big change for __vm_enough_memory() 
> 
>      0.27            -0.3        0.00        pp.self.__vm_enough_memory
> 
> I post the full compare result in the end.

I don’t really see what that means exactly, but I suppose the warning is there for so long and no one seems notice much trouble (or benefit) because of it, so I think you will probably need to come up with a proper justification to explain why it is a trouble now, and how your patchset suddenly start to trigger the warning as well as why it is no better way but to suffer this debuggability regression (probably tiny but still).

^ permalink raw reply	[flat|nested] 16+ messages in thread
end of thread, other threads:[~2020-06-04  1:38 UTC | newest]

Thread overview: 16+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-05-29  1:06 [PATCH v4 0/4] make vm_committed_as_batch aware of vm overcommit policy Feng Tang
2020-05-29  1:06 ` [PATCH v4 1/4] proc/meminfo: avoid open coded reading of vm_committed_as Feng Tang
2020-05-29  1:06 ` [PATCH v4 2/4] mm/util.c: make vm_memory_committed() more accurate Feng Tang
2020-06-03 13:35   ` Michal Hocko
2020-06-03 14:28   ` Andi Kleen
2020-06-04  1:38     ` Feng Tang
2020-05-29  1:06 ` [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check Feng Tang
2020-05-29  2:49   ` Qian Cai
2020-05-29  5:37     ` Feng Tang
2020-06-02  3:37     ` Feng Tang
2020-05-29  1:06 ` [PATCH v4 4/4] mm: adjust vm_committed_as_batch according to vm overcommit policy Feng Tang
2020-06-03 13:38   ` Michal Hocko
2020-06-02  4:02 [PATCH v4 3/4] mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check Qian Cai
2020-06-03  9:48 ` Feng Tang
2020-06-03 11:51   ` Qian Cai
2020-06-03 13:36   ` Michal Hocko

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