Question about process.c

[Anton Ivanov]

OK.

I will park it until I have a large timeslot to play with it and analyze
it properly.

A.

On 01/05/18 15:36, Richard Weinberger wrote:
> Anton,
>
> Am Dienstag, 1. Mai 2018, 15:48:48 CEST schrieb Anton Ivanov:
>> arch/um/kernel/process.c line 140 invokes force_flush_all();
>>
>> which does a full tlb flush with the force flag set.
>>
>> I did a quick test and I did not notice any breakage if we replace that 
>> with a quickly hacked together new function called gentle_flush_all() 
>> which does the same but without setting the force flag in the tlb routines.
>>
>> At the same time I got 10% or better speedup for fork-heavy things like 
>> startup, etc.
>>
>> In fact, this is one of the last remaining major performance bugbears. 
>> We got most of the other stuff to a very reasonable standard - 
>> networking, disk, etc is not that far off from let's say qemu and the 
>> overall "slowness" is now mostly down to the huge cost of fork/exec.
>>
>> Example (ab)using busybox. Busybox executes cat passed as the find -exec 
>> argument internally shortcutting to its applet. As a result there is no 
>> cost of fork/exec incurred when we run a find -exec cat {} via busybox. 
>> Normal find executes /bin/cat instead.
>>
>> This allows us to compare and attribute the cost of fork/exec in scripts 
>> as well as gives us a benchmark to see the effect of any changes.
>>
>> Bare metal:
>>
>> aivanov at amistad:/var/autofs/local/src/linux-work/linux-submit$ time 
>> busybox find /mnt/usr -type f -exec cat {} > /dev/null \;
>>
>> real    0m16.511s
>> user    0m12.672s
>> sys    0m4.001s
>> aivanov at amistad:/var/autofs/local/src/linux-work/linux-submit$ time find 
>> /mnt/usr -type f -exec cat {} > /dev/null \;
>>
>> real    0m25.329s
>> user    0m16.397s
>> sys    0m9.185s
>>
>> UML:
>>
>> root at uml-switch:~# time busybox find /usr -type f -exec cat {} > 
>> /dev/null \;
>>
>> real    0m11.447s
>> user    0m0.000s
>> sys    0m8.820s
>> root at uml-switch:~# time find /usr -type f -exec cat {} > /dev/null \;
>>
>> real    7m8.228s
>> user    0m0.000s
>> sys    6m42.780s
>>
>> The filesystem is identical in both cases. The experiment is not 
>> perfectly "clean" as i am doing other stuff on my laptop, but it is 
>> reasonably indicative
>>
>> In fact, in the longer term I would really like to somehow speed-up most 
>> of the stuff tlb.c It is painfully slow at present.
> Hmmm, not sure. I fear without going deep into the UML history we cannot know.
> I think we have to do a full flush for security reasons.
>
> Thanks,
> //richard
>