Re: clock_gettime.2: _COARSE clocks are not always faster...
On 04/18/2016 04:12 PM, Michael Kerrisk (man-pages) wrote:
> Hello Rasmus,
>
> On 04/09/2016 05:50 PM, Rasmus Villemoes wrote:
>> Hi Michael
>>
>> The other day, I was curious how the vdso was implemented on ppc, and I
>> noted that neither ppc32 or ppc64 handle the _COARSE versions of
>> CLOCK_{REALTIME,MONOTONIC} in the vdso, so they fall back to an actual
>> syscall. And sure enough, measuring CLOCK_MONOTONIC
>> vs. CLOCK_MONOTONIC_COARSE shows that the latter has three times as much
>> overhead as the former.
>>
>> Whether it's worth adding a note to the man page is up to you.
>
> I think it's useful to do so. For both *COARSE flags, I added:
>
> [[
> Requires per-architecture support, and probably also architecture support
> for this flag in the vdso(7).
> ]]
And in the PowerPC sections of vdso(7), I added:
The CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE clocks are
not supported by the __kernel_clock_getres and __ker‐
nel_clock_gettime interfaces; the kernel falls back to the real
system call.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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Re: clock_gettime.2: _COARSE clocks are not always faster...
Hello Rasmus,
On 04/09/2016 05:50 PM, Rasmus Villemoes wrote:
> Hi Michael
>
> The other day, I was curious how the vdso was implemented on ppc, and I
> noted that neither ppc32 or ppc64 handle the _COARSE versions of
> CLOCK_{REALTIME,MONOTONIC} in the vdso, so they fall back to an actual
> syscall. And sure enough, measuring CLOCK_MONOTONIC
> vs. CLOCK_MONOTONIC_COARSE shows that the latter has three times as much
> overhead as the former.
>
> Whether it's worth adding a note to the man page is up to you.
I think it's useful to do so. For both *COARSE flags, I added:
[[
Requires per-architecture support, and probably also architecture support
for this flag in the vdso(7).
]]
Thanks!
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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clock_gettime.2: _COARSE clocks are not always faster...
Hi Michael
The other day, I was curious how the vdso was implemented on ppc, and I
noted that neither ppc32 or ppc64 handle the _COARSE versions of
CLOCK_{REALTIME,MONOTONIC} in the vdso, so they fall back to an actual
syscall. And sure enough, measuring CLOCK_MONOTONIC
vs. CLOCK_MONOTONIC_COARSE shows that the latter has three times as much
overhead as the former.
Whether it's worth adding a note to the man page is up to you.
Best,
Rasmus
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