Question about process.c

[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.

A.

On 05/01/18 14:21, Richard Weinberger wrote:
> Am Dienstag, 1. Mai 2018, 15:01:54 CEST schrieb Anton Ivanov:
>> Hi all, hi Richard,
>>
>> I have a question - is it really necessary to pass a "force" flag to a
>> tlb flush before doing a fork (in process.c)?
> Where exactly?
>
> Thanks,
> //richard
>

-- 
Anton R. Ivanov

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