[tor-dev] gettimeofday() Syscall Issues

[Adrian Chadd]

What's their benchmark call?

What about clock_gettime() versus gettimeofday() ?




-adrian


On 2 January 2015 at 06:09, grarpamp <grarpamp at gmail.com> wrote:
> Some recent FreeBSD related questions in this app area.
>
>
> ---------- Forwarded message ----------
> From: Yawning Angel <yawning at schwanenlied.me>
> Date: Fri, Jan 2, 2015 at 5:20 AM
> Subject: Re: [tor-dev] gettimeofday() Syscall Issues
> To: tor-dev at lists.torproject.org
>
>
> On Thu, 01 Jan 2015 23:42:42 -0500
> Libertas <libertas at mykolab.com> wrote:
>
>> The first two account for the bulk of the calls, as they are in the
>> core data relaying logic.
>>
>> Ultimately, the problem seems to be that the caching is very weak. At
>> most, only half of the calls to tor_gettimeofday_cached_monotonic()
>> use the cache. It appears in the vomiting print statements that
>> loading a single simple HTML page
>> (http://www.openbsd.org/faq/ports/guide.html to be exact) will cause
>> >30 gettimeofday() syscalls. You can imagine how that would
>> >accumulate for an exit carrying 800 KB/s if the caching
>> doesn't improve much with additional circuits.
>
> So while optimization is cool and all, I'm not seeing why this
> specifically is the underlying issue.
>
> Each cell can contain 498 bytes of user payload.  Looking at things
> simplistically this is 800 KiB/s -> 1644 cells/sec, leaving you with
> approximately 608 microseconds of processing time per cell.
>
> On my i5-4250U box, gettimeofday() takes 22 ns on Linux, and 2441 ns on
> FreeBSD.  I'm not sure how accurate the FreeBSD results are as it was
> in a VirtualBox VM (getpid() on the same VM takes 124 ns).  If someone
> has a OpenBSD box they should benchmark gettimeofday() and see how long
> the call takes.
>
> Taking the FreeBSD case (since we know that tor works fine on Linux), a
> single gettimeofday() call takes approximately, 0.39% of the per-cell
> processing budget.
>
> For reference (assuming gettimeofday() in *BSD really is this shit
> performance wise), 7000 calls to gettimeofday() is 17.09 ms worth of
> calls.
>
> The clock code in tor does need love, so I wouldn't object to cleanup,
> but I'm not sure it's in the state where it's causing the massive
> performance degradation that you are seeing.
>
> Regards,
>
> --
> Yawning Angel
>
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