ETW traces record a wealth of information about how a Windows system is behaving. When analyzing a new and unknown problem there is no replacement for loading the trace into WPA and following the clues to a solution. The thrill of the hunt and the creative challenge of finding a visualization that will reveal the root cause never gets old (too nerdy? Sorry – I do enjoy this).
But sometimes you want to extract some commonly found piece of information from multiple traces, and doing this manually is tedious and error prone.
I recently hit some multi-minute delays on my workstation. After investigating I found that the problem was due to a lock being held for five minutes, and during that time the lock-holder was mostly just spinning in a nine-instruction loop.
Coming up with a good title for my blog posts is critical but I immediately realized that the obvious title of “48 processors blocked by nine instructions” was taken already by a post from less than a month earlier. The number of processors blocked is different, and the loop is slightly longer but, really, it’s just deja vu all over again. So, while I will explain the new issue that I found I first want to discuss the question of why this keeps happening.
Why does this happen?
Roughly speaking it’s due to an observation which I’m going to call Dawson’s first law of computing: O(n^2) is the sweet spot of badly scaling algorithms: fast enough to make it into production, but slow enough to make things fall down once it gets there.
I’ve recently started using heap snapshots on Windows to track heap allocations. I was able to use heap snapshots to record call stacks for all outstanding allocations in Chrome’s browser process over a full two weeks, letting me account for pretty much every byte of memory consumed.
Since then I have used heap snapshots to find wasteful memory usage in the Windows heap, a memory leak in a security tool injected into Chrome, and many details of Chrome’s memory usage that I was not previously aware of.
I first read about heap snapshots here. This page gives the mechanics of how to record a heap snapshot but it spends very little time explaining what heap snapshots are or how to use them effectively.
I seem to have a habit of writing about super powerful machines whose many cores are laid low by misuse of locks. So. Yeah. It’s that again.
But this one seems particularly impressive. I mean, how often do you have one thread spinning for several seconds in a seven-instruction loop while holding a lock that stops sixty-three other processors from running. That’s just awesome, in a horrible sort of way.
Contrary to popular belief I don’t actually have a machine with 64 logical processors, and I’ve never seen this particular problem. But a friend hit this problem,
nerd-sniped me asked for help, and I decided it was interesting enough to look at. They sent me an ETW trace that contained enough information for me to craft a tweet-for-help which resolved the issue swiftly.
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So many possible introductions to this one:
- Windows 7: Sheesh, I sure am slow at creating processes
- Windows 10: Hold my beer…
Or how about:
- A) How long does CreateProcess take on Windows?
- B) How long would you like it to take?
- A) You mean you can make it as fast as I want?
- B) No, I can make it as *slow* as you want
O(n^2) algorithms that should be linear are the best.
Note that, despite breathless and click-baity claims to the contrary, the performance of Chrome and Chromium was never affected by this bug. Only Chromium’s tests were affected, and that slowdown has been 100% mitigated.
CFG ended up being a big part of this issue, and eight months earlier I had hit a completely unrelated CFG problem, written up here.
I often find odd performance issues all on my own, but sometimes they are given to me. So it was when I returned from vacation to find that I’d been CCed on an interesting looking bug. Vivaldi had reported “Unit test performance much worse on Win10 than Win7”. unit_tests were taking 618 seconds on Win10, but just 125 seconds on Win7.
Update, April 23, 2019: Microsoft received the initial “anomaly” report on the 15th, the repro steps on the 21st, and announced a fix today. Quick work!
By the time I looked at the bug it was suspected that CreateProcess running slowly was the problem. My first guess was that the problem was UserCrit lock contention caused by creating and destroying default GDI objects. Windows 10 made these operations far more expensive, I’d already written four blog posts about the issues that this causes, and it fit the symptoms adequately well.