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

Ok, I will withdraw this PR, but...

> 2. As Kim points out, you may be the first to actually use the bogomips
> code with this change, which is even more scarey!

No, the bogomips might be used in some systems.

https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/rdtsc_x86.cpp#L102-L119

For example, if the user runs JVM with -XX:+UseFastUnorderedTimeStamps on the machine which is not supported invariant TSC (e.g. virtualization guest), bogomips will be used. We can get following flight record on it:


jdk.CPUTimeStampCounter {
jdk.CPUTimeStampCounter {
  startTime = 17:11:52.351
  fastTimeEnabled = true
  fastTimeAutoEnabled = false
  osFrequency = 1000000000
  fastTimeFrequency = 3792659755
}


But we can see following warnings, so most of user can understood it is unstable:


The hardware does not support invariant tsc (INVTSC) register and/or cannot guarantee tsc synchronization between sockets at startup.
Values returned via rdtsc() are not guaranteed to be accurate, esp. when comparing values from cross sockets reads. Enabling UseFastUnorderedTimeStamps on non-invariant tsc hardware should be considered experimental.


I know it is corner case, so we can say bogomips is not widely used.

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

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