From: "Zack Weinberg" <zack@owlfolio.org>
To: "GNU libc development" <libc-alpha@sourceware.org>
Subject: Re: bug fix for hp-timing.h (aarch64)
Date: Thu, 12 Jan 2023 10:11:46 -0500 [thread overview]
Message-ID: <a9903450-3f81-493c-a8cf-1ca68ccb1ae0@app.fastmail.com> (raw)
In-Reply-To: <PAWPR08MB8982320110E6CC2FF66F5F4E83FC9@PAWPR08MB8982.eurprd08.prod.outlook.com>
On Wed, Jan 11, 2023, at 5:49 PM, Wilco Dijkstra via Libc-alpha wrote:
>> - (Diff) = ((End) - (Start)) * (UINT64_C(1000000000) / freq); \
>> + (Diff) = (((End) - (Start)) * UINT64_C(1000000000)) / freq; \
>>
>> This avoids using floating-point arithmetic but should still do the rescale correctly.
>
> However this will overflow after about 16 seconds - and that's short
> even for basic benchmarking.
I'm surprised to hear you say that. To me, 16 seconds is an absurdly _long_ interval for use of hp-timing.h. For anything longer than a few tens of milliseconds, the overhead of two system calls is negligible, but both CPU frequency changes and getting descheduled in the middle of the measurement become serious concerns. So it seems more appropriate to me to use clock_gettime with a per-process or per-thread clock ID for measurements on the scale of tens of seconds.
This isn't a sustained objection to the patch, I just want to understand where you're coming from.
zw
next prev parent reply other threads:[~2023-01-12 15:12 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-11 22:49 Wilco Dijkstra
2023-01-12 14:38 ` Tang, Jun
2023-01-12 18:07 ` Wilco Dijkstra
2023-01-12 18:54 ` Zack Weinberg
2023-01-12 20:32 ` Wilco Dijkstra
2023-01-12 20:51 ` Noah Goldstein
2023-01-16 16:33 ` Wilco Dijkstra
2023-01-16 17:01 ` Noah Goldstein
2023-01-16 18:35 ` Wilco Dijkstra
2023-01-12 15:11 ` Zack Weinberg [this message]
-- strict thread matches above, loose matches on Subject: below --
2023-01-11 16:44 Tang, Jun
2023-01-11 17:22 ` Zack Weinberg
2023-01-31 14:47 ` Tang, Jun
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