From: wangyanan (Y)
Sent: 19 April 2021 07:40
Hi Paolo,
On 2021/4/17 21:23, Paolo Bonzini wrote:
On 30/03/21 10:08, Yanan Wang wrote:
In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can also shield possiable impact of NTP, which can provide more robustness.
Suggested-by: Vitaly Kuznetsovvkuznets@redhat.com Signed-off-by: Yanan Wangwangyanan55@huawei.com Reviewed-by: Ben Gardonbgardon@google.com Reviewed-by: Andrew Jonesdrjones@redhat.com
I'm not sure about this one, is the effect visible?
In practice, difference between results got with CLOCK_MONOTONIC and CLOCK_MONOTONIC_RAW actually is too little to be visible. But if just in theory, CLOCK_MONOTONIC_RAW can ensure time results of the compared tests are based on the same local oscillator frequency, which is not subject to possible NTP frequency adjustment. Change in this patch seems like a bit of optimization.
The real annoyance is when NTP is realigning the local clock. This typically happens after boot - but can take quite a few minutes (don't think it can quite get to an hour). (I think something similar is caused by leap seconds.)
During this period CLOCK_MONOTONIC can run at a significantly different rate from 'real time'. This may not matter for timing self tests, but is significant for RTP audio.
The problem there is that you want the NTP corrected time during 'normal running' because the small correction (for crystal error) is useful.
But the kernel HR timers are only defined for CLOCK_MONOTONIC and the userspace requests for CLOCK_MONOTONIC_RAW are likely to be real system calls.
What you really want is a clock whose frequency is adjusted by NTP but doesn't have the NTP offset adjuctments. In reality this ought to be CLOCK_MONOTONIC.
David
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