On Sat, May 28, 2016 at 12:19 PM, Antti Kantee <[email protected]> wrote:
> On 27/05/16 14:04, George Dunlap wrote:
>>
>> On Fri, May 27, 2016 at 1:17 PM, Antti Kantee <[email protected]> wrote:
>>>
>>> Well, try this hacky patch. It's not correct by any stretches of
>>> imagination, nor am I promising to accept something like it into Rumprun,
>>> but if you can repeat my results, at least we've narrowed down the
>>> problem.
>>> In my testing the clock_gettime-to-clock_gettime delay for a 1ns sleep on
>>> KVM is ~8us with this patch.
>>>
>>> (btw, the init code might not in correct order as indicated by the
>>> comment,
>>> try to hack around it somehow if that happens)
>>
>>
>> Hmm, it didn't work as advertised; the code seems to have run, but the
>> same issue remains:
>
>
> Then I don't know. It made a difference for me. There's something to be
> said about the scientific process of posting the actual code to allow others
> to repeat the experiment. Here's mine:
>
> === snip ===
> #include <sys/types.h>
>
> #include <stdio.h>
> #include <time.h>
> #include <unistd.h>
>
> int
> main()
> {
> struct timespec ts, ts1, ts2;
>
> ts.tv_sec = 0;
> ts.tv_nsec = 1000;
>
> clock_gettime(CLOCK_REALTIME, &ts1);
> nanosleep(&ts, NULL);
> clock_gettime(CLOCK_REALTIME, &ts2);
>
> timespecsub(&ts2, &ts1, &ts);
> printf("%ld\n", ts.tv_nsec);
> }
> === snip ===
Ah -- right; it turns out that I wasn't actually using nanosleep()
inside that "nsleep() call, I was using pselect. (I think that's
because I had determined that would be the most portable, but I can't
remember why now.)
In any case, with nanosleep() and this patch, then I'm getting nice
small deltas:
{ "Now":1000782714, "Mops":0, "MaxDelta":0 }
{ "Now":2055607406, "Mops":300, "MaxDelta":25834 }
{ "Now":3057985717, "Mops":580, "MaxDelta":8441 }
{ "Now":4060254047, "Mops":860, "MaxDelta":6169 }
{ "Now":5062553556, "Mops":1140, "MaxDelta":10956 }
{ "Now":6068979087, "Mops":1420, "MaxDelta":5955 }
Thanks for your help.
>> It's a bit strange that NetBSD clips the sleep time to the clock HZ --
>> that seems like it's probably an artifact of a bygone age.
>
> "patches welcome", and all that.
I'll see what I can do. :-)
-George
