RFR: 8268228: TSC is not used for CPUTimeStampCounter on AMD processor [v2]

[Yasumasa Suenaga]

On Fri, 4 Jun 2021 05:24:15 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> I ran JVM on Ryzen 3300X, and I got following `jdk.CPUTimeStampCounter` event.
>> 
>> 
>> jdk.CPUTimeStampCounter {
>>   startTime = 10:41:14.993
>>   fastTimeEnabled = false
>>   fastTimeAutoEnabled = true
>>   osFrequency = 1000000000
>>   fastTimeFrequency = 1000000000
>> }
>> 
>> 
>> I confirmed 3300X supports Invariant TSC (so `fastTimeAutoEnabled` is set to `true`), however it does not seem to be used (`fastTimeEnabled` is `false`).
>> 
>> Frequency is come from brand string from CPUID (e.g. "Intel(R) Core(TM) i3-8145U CPU @ 2.10GHz"). However AMD processor (Ryzen at least) does not have it ("AMD Ryzen 3 3300X 4-Core Processor").
>> Fortunately rdtsc_x86.cpp can calculate the frequency like bogomips. We should fallback to it if we cannot get the frequency even if invariant TSC is supported.
>> 
>> After this change, I got following `jdk.CPUTimeStampCounter` event. Base clock of Ryzen 3 3300X is 3.8GHz, so `fastTimeFrequency` looks good.
>> 
>> 
>> jdk.CPUTimeStampCounter {
>>   startTime = 10:33:52.884
>>   fastTimeEnabled = true
>>   fastTimeAutoEnabled = true
>>   osFrequency = 10000000 Hz
>>   fastTimeFrequency = 3792929124 Hz
>> }
>> 
>> 
>> This problem is not only for JFR. I confirmed `Rdtsc` class is used in ticks.cpp , and it relates to GC code at least.
>
> Yasumasa Suenaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix comments

To be honest I like TSC for calculating short time difference because execution cost is much lesser than syscalls. So I want to use it on AMD processor especially with JFR - it requires event duration which is calculated by TSC or by timestamp. I think there is not so big problem for this purpose.

However, in general, if TSC should not be used in HotSpot, I will withdraw this PR and will close JBS.

-------------

PR: https://git.openjdk.java.net/jdk/pull/4350