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

[Davidlohr Bueso]

On Fri, 30 Mar 2018, Eric W. Biederman wrote:

>Davidlohr Bueso <dave at stgolabs.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.

Yeah, sorry that was a typo. Unlike the second benchmark I didn't have
this one automated but it's always the vanilla kernel that outperforms
the dirty.

Thanks,
Davidlohr
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