GNU C Library version 2.39 [LWN.net]

GNU C Library version 2.39

Posted Feb 8, 2024 4:40 UTC (Thu) by intelfx (subscriber, #130118)
In reply to: GNU C Library version 2.39 by farnz
Parent article: GNU C Library version 2.39

Last time I checked, per-bdi writeback limits were totally nonfunctional even when set manually. In the kernel, I think there even were some references to auto-tuning these limits, but that behavior is also nowhere to be seen.

Could you please clarify how do I make them work? :-)

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GNU C Library version 2.39

Posted Feb 8, 2024 10:28 UTC (Thu) by farnz (subscriber, #17727) [Link]

Tested on 6.6.6 and 6.7.3 kernels, on a Fedora 39 system. I have a slow USB device as /dev/sda, which is therefore bdi 8:0. To restrict it to 1 second of dirty data, I need to run two commands as root:

echo 1 > /sys/class/bdi/8:0/strict_limit
echo $((4 * 1024 * 1024)) > /sys/class/bdi/8:0/max_bytes

Per the documentation for bdi limits, the first command tells the kernel that this device's limits must be respected even if the global background dirty limit isn't reached; my system has a global background dirty limit around 8 GiB, so without this, any limit I set below 8 GiB is ignored.

The second sets the actual limit - in this case, 1 second of writes to the device, which is 4 MiB of data. You can see why strict limits matters here, though - without strict limits, the global background dirty limit would push me to 8 GiB of data before even checking the per-bdi limits, unless I had a lot of writes in progress to other devices. And I want relatively large limits for the global limits, since my laptop has a big battery, and when I build a Yocto image, I've often got many readers in parallel that need time on the NVMe drive, along with writes that can be delayed, and I'd prefer the writes to wait so that I'm blocking on CPU, not on I/O.