linux-kernel - Re: [REVIEW][PATCH 11/11] ipc/sem: Fix semctl(..., GETPID, ...) between pid namespaces

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Date:   Fri, 30 Mar 2018 15:12:10 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     manfred@...orfullife.com,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        khlebnikov@...dex-team.ru, prakash.sangappa@...cle.com,
        luto@...nel.org, akpm@...ux-foundation.org, oleg@...hat.com,
        serge.hallyn@...ntu.com, esyr@...hat.com, jannh@...gle.com,
        linux-security-module@...r.kernel.org,
        Pavel Emelyanov <xemul@...nvz.org>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>
Subject: Re: [REVIEW][PATCH 11/11] ipc/sem: Fix semctl(..., GETPID, ...) between pid namespaces

Davidlohr Bueso <dave@...olabs.net> writes:

> I ran this on a 40-core (no ht) Westmere with two benchmarks. The first
> is Manfred's sysvsem lockunlock[1] program which uses _processes_ to,
> well, lock and unlock the semaphore. The options are a little
> unconventional, to keep the "critical region small" and the lock+unlock
> frequency high I added busy_in=busy_out=10. Similarly, to get the
> worst case scenario and have everyone update the same semaphore, a single
> one is used. Here are the results (pretty low stddev from run to run)
> for doing 100,000 lock+unlock.
>
> - 1 proc:
>   * vanilla
> 	total execution time: 0.110638 seconds for 100000 loops
>   * dirty
> 	total execution time: 0.120144 seconds for 100000 loops
>
> - 2 proc:
>   * vanilla
> 	total execution time: 0.379756 seconds for 100000 loops
>   * dirty
> 	total execution time: 0.477778 seconds for 100000 loops
>
> - 4 proc:
>   * vanilla
> 	total execution time: 6.749710 seconds for 100000 loops
>   * dirty
> 	total execution time: 4.651872 seconds for 100000 loops
>
> - 8 proc:
>   * vanilla
>        total execution time: 5.558404 seconds for 100000 loops
>   * dirty
> 	total execution time: 7.143329 seconds for 100000 loops
>
> - 16 proc:
>   * vanilla
> 	total execution time: 9.016398 seconds for 100000 loops
>   * dirty
> 	total execution time: 9.412055 seconds for 100000 loops
>
> - 32 proc:
>   * vanilla
> 	total execution time: 9.694451 seconds for 100000 loops
>   * dirty
> 	total execution time: 9.990451 seconds for 100000 loops
>
> - 64 proc:
>   * vanilla
> 	total execution time: 9.844984 seconds for 100032 loops
>   * dirty
> 	total execution time: 10.016464 seconds for 100032 loops
>
> Lower task counts show pretty massive performance hits of ~9%, ~25%
> and ~30% for single, two and four/eight processes. As more are added
> I guess the overhead tends to disappear as for one you have a lot
> more locking contention going on.

Can you check your notes on the 4 process case?  As I read the 4 process
case above it is ~30% improvement.  Either that is a typo or there is the
potential for quite a bit of noise in the test case.


Eric