Viewing profile — jerrinot

clock_gettime() goes through the vDSO shim, but whether it avoids a syscall depends on the clock ID and (in some cases) the clock source. For thread-specific CPU user time, the vDS…

You raise a fair point about the percentiles. Those are reported as point estimates without confidence intervals and the implied precision overstates what system clock can deliver.…

Hi Jonas, thanks for the work on OpenJDK and the post! I swear I hadn't seen your blog :) I finished my draft around Christmas and it’s been in the queue since. Great minds think a…

In a system with green threads, you typically want the CPU time of the fiber or tasklet rather than the carrier thread. In that case, you have to ask the scheduler, not the kernel.…

Courtesy of Brendan Gregg and his flamegraph.pl scripts: https://github.com/brendangregg/FlameGraph Normally, I use the generator included in async-profiler. It produces interactiv…

no problem at all, I was confused too when I saw the profile for the first time.

That's a brilliant trick. The setup overhead and permission requirements for perf_event might be heavy for arbitrary threads, but for long-lived threads it looks pretty awesome! Th…

Exactly this.

Only for some clocks (CLOCK_MONOTONIC, etc) and some clock sources. For VIRT/SCHED, the vDSO shim still has to invoke the actual syscall. You can't avoid the kernel transition when…

I was unsure whether to post it or not so I am glad you found it useful!

Fair point. These were run on a standard dev workstation under load, which may account for the noise. I haven't done a deep dive into the outliers yet, but the distribution definit…

Author here. After my last post about kernel bugs, I spent some time looking at how the JVM reports its own thread activity. It turns out that "What is the CPU time of this thread?…

I have no practical experience with bpftrace, so it did not occur to me. I'll give it a try and perhaps there's gonna be a 2nd part of this investigation.

It's much tougher when it's so hard to reproduce. Perhaps the NMI watchdog could help? https://docs.kernel.org/admin-guide/lockup-watchdogs.html

Wow, someone is actually reading the article in detail, that's a good feeling! In C, the != operator has higher precedence than the || operator. That said, extra parentheses never …

Thanks for the kind words! Heatmaps are amazing for pattern spotting. I also use them when hunting irregular hiccups or outliers. More people should know about this feature.

Author here. I've always been kernel-curious despite never having worked on one myself. Consider this either a collection of impractical party tricks or a hands-on way to get a fee…

Snowflake did not acquire RP after all.

I expect candidates to suggest similar optimisations, but I felt it was unnecessary for the article itself.

That’s a neat trick, albeit with limited applicability given the very narrow range. Thanks for sharing!

That's a very good question. A proper compiler engineer would know, but I will do my best to find something and report back. Edit: I could not find any pass with a pattern matching…

Hi, author here. My superpower is spending unreasonable amounts of time researching things with no practical purpose. Occasionally I blog about it - as a warning to others.