[PATCH] Use clock_gettime(REAL_FAST) on DragonFlyBSD
mdounin at mdounin.ru
Thu Jun 2 18:21:15 UTC 2016
On Thu, Jun 02, 2016 at 11:19:26PM +0800, Sepherosa Ziehau wrote:
> On Thu, Jun 2, 2016 at 11:05 PM, Maxim Dounin <mdounin at mdounin.ru> wrote:
> > Hello!
> > On Thu, Jun 02, 2016 at 08:48:19PM +0800, Sepherosa Ziehau wrote:
> >> On DragonflyBSD only clock_gettime(*_FAST) avoids system call and time
> >> counter read cost (kinda like Linux VDSO). This gives some
> >> improvement for performance and reduces latency.
> > At least FreeBSD also has clock_gettime(*_FAST), from where it was
> > taken to DragonFly BSD. And Linux 2.6.32+ has
> > clock_gettime(*_COARSE).
> > We do have plans to use monotonic versions, see
> > https://trac.nginx.org/nginx/ticket/189.
> Yeah, if we can use clock_gettime(REAL_FAST/REAL_COARSE) that would be great :)
> Should I submit another patch for it? I think the clockid can be put
> into the config file.
> I patched the Dfly specifically, mainly because its gettimeofday is
> still a syscall and read time counter. While both FreeBSD-current and
> Linux (which seems to have a sysctl to enable) has VDSO
I don't really think we really should introduce
clock_gettime(CLOCK_REALTIME*) instead of gettimeofday(). It's
not really needed as gettimeofday() isn't called often.
Though we can consider doing this as a part of
clock_gettime(CLOCK_MONOTONIC) work as we are going to add
appropriate configure tests anyway.
> >> My testing setup consists two clients and one nginx (recent head code)
> >> server, two clients connected to the nginx server through two 10Ge
> >> (82599). 15K concurrent connections from each client; client runs a
> >> modified version of wrk and each connection only carries one request
> >> for an 1KB file.
> >> Performance goes from 183Kreqs/s to 184Kreqs/s (admittedly pretty
> >> minor). Average latency reduces 2ms (98ms -> 96ms). Latency stdev
> >> reduces 6ms (198ms -> 192ms).
> > This doesn't look statistically significant. Have you tried
> > something like ministat(1) to test the results?
> I have wrk --latency data:
> Latency Distribution
> 50% 26.03ms
> 75% 73.95ms
> 90% 285.33ms
> 99% 1.04s
> Latency Distribution
> 50% 22.63ms
> 75% 77.42ms
> 90% 278.79ms
> 99% 1.01s
> Admittedly, kinda like acceptable measurement errors :P.
Doesn't look like a statistically significant difference for me,
at least in the absense of a margin of error.
If you think there is a difference, please try ministat, as
already suggested. Not sure it's available on DragonFly BSD, but
you can easily grab it here:
More information about the nginx-devel