AMD Updates Linux Patches For Lowering Idle Exit Latency

[phoronix]

Phoronix: AMD Updates Linux Patches For Lowering Idle Exit Latency

Last month an AMD engineer began posting Linux kernel patches so the kernel prefers the MWAIT instruction over HALT for lowering the CPU idle exit latency. Preferring MWAIT over HALT has been something Intel CPUs on Linux have preferred going back to the Core 2 days and indeed with modern AMD CPUs there is significant advantages to lowering the exit latency in doing so for the idle code. This morning the latest iteration of the work was posted...

https://www.phoronix.com/scan.php?page=news_item&px=AMD-Prefer-MWAIT-v3

[Anux]

Is there something switched up? MWAIT values are larger than HALT.

[Michael]

Is there something switched up? MWAIT values are larger than HALT.

That graphic in the article is of the context switching perf, not latency. See the mailing list post for the other latency numbers.

[Anux]

Ah nice and there are also nano seconds in the patch. THX

[cyring]

It's a nice move from AMD; using Zen architectures, having the number of cycles elapsed in deep idle states like Core and Package C6, (Intel is providing them through MSR registers) would be awesome.

For unfound reasons, yet, my observed energy measurements in CoreFreq, using RAPL registers, reveal a per Core power increased when the kernel loop is idling in MONITOR/MWAIT versus HALT or I/O wait.


Recommended to check the associated bit state in HWCR before executing MWAIT or Processor freeze.

[Spacefish]

For unfound reasons, yet, my observed energy measurements in CoreFreq, using RAPL registers, reveal a per Core power increased when the kernel loop is idling in MONITOR/MWAIT versus HALT or I/O wait.

As far as i understand it, with MWAIT you can specify a "C-state" to enter, with HLT C0 is entered.. Probably a tradeoff between wakeup time and power saving. The MWAIT patch might not enter such a deep sleep state than the HLT instruction did before?

[cyring]

As far as i understand it, with MWAIT you can specify a "C-state" to enter, with HLT C0 is entered.. Probably a tradeoff between wakeup time and power saving. The MWAIT patch might not enter such a deep sleep state than the HLT instruction did before?

Trying various hints did not make a difference.
But I noticed that the MONITOR/MWAIT power overhead is barely the same as Core C6 state (CC6) disabled

• HALT + CC6 enabled: 0.1W (Cores) and 16W (package)
• HALT + CC6 disabled: 4W (Cores) and 32W (package)
• MONiTOR/MWAIT + CC6 enabled = 4W (Cores) and 32W (package)
• MONITOR/MWAIT + CC6 disabled = same as above
• I/O wait follows HALT results

2022-05-11-031050_644x1012_scrot.png

[MorrisS.]

how to know if mwait or halt is applied? Which kernel will integrate this patch? Which cpus will apply this patch on?