Making my machine stay responsive when writing to USB drives

January 17, 2017

Yesterday I talked about how writing things to USB drives made my machine not very responsive, and in a comment Nolan pointed me to LWN's The pernicious USB-stick stall problem. According to LWN's article, the core problem is an excess accumulation of dirty write buffers, and they give some VM system sysctls that you can use to control this.

I was dubious that this was my problem, for two reasons. First, I have a 16 GB machine and I barely use all that memory, so I thought that allowing a process to grab a bit over 3 GB of them for dirty buffers wouldn't make much of a difference. Second, I had actually been running sync frequently (in a shell loop) during the entire process, because I have sometimes had it make a difference in these situations; I figured frequent syncs should limit the amount of dirty buffers accumulating in general. But I figured it couldn't hurt to try, so I used the dirty_background_bytes and dirty_bytes settings to limit this to 256 MB and 512 MB respectively and tested things again.

It turns out that I was wrong. With these sysctls turned down, my machine stayed quite responsive for once, despite me doing various things to the USB flash drive (including things that had had a terrible effect just yesterday). I don't entirely understand why, though, which makes me feel as if I'm doing fragile magic instead of system tuning. I also don't know if setting these down is going to have a performance impact on other things that I do with my machine; intuitively I'd generally expect not, but clearly my intuition is suspect here.

(Per this Bob Plankers article, you can monitor the live state of your system with egrep 'dirty|writeback' /proc/vmstat. This will tell you the number of currently dirty pages and the thresholds (in pages, not bytes). I believe that nr_writeback is the number of pages actively being flushed out at the moment, so you can also monitor that.)

PS: In a system with drives (and filesystems) of vastly different speeds, a global dirty limit or ratio is a crude tool. But it's the best we seem to have on Linux today, as far as I know.

(In theory, modern cgroups support the ability to have per-cgroup dirty_bytes settings, which would let you add extra limits to processes that you knew were going to do IO to slow devices. In practice this is only supported on a few filesystems and isn't exposed (as far as I know) through systemd's cgroups mechanisms.)

Written on 17 January 2017.
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Last modified: Tue Jan 17 00:36:09 2017
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