#22349: OpenWRT wrong adjustment of fq_codel defaults
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Reporter: brouer@… | Owner: developers
Type: defect | Status: new
Priority: highest | Milestone: Bugs Paradise
Component: kernel | Version: Trunk
Keywords: |
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This is an important fix for OpenWRT, please read!
OpenWRT changed the default fq_codel sch->limit from 10240 to 1024,
without also adjusting q->flows_cnt (number of buckets). Eric Dumazet
suggest to have at least 8 packets per bucket, to let Codel have a chance
to trigger. Thus you must also adjust the buckets (q->flows_cnt) for this
not to break. (With 1024 / 8 adjust it to 128)
Problematic OpenWRT commit in question:
http://git.openwrt.org/?p=openwrt.git;a=patch;h=12cd6578084e
12cd6578084e ("kernel: revert fq_codel quantum override to prevent it
from causing too much cpu load with higher speed (#21326)")
I also highly recommend you cherry-pick this very recent commit:
net-next: 9d18562a2278 ("fq_codel: add batch ability to fq_codel_drop()")
https://git.kernel.org/davem/net-next/c/9d18562a227
https://patchwork.ozlabs.org/patch/617307/
This should fix very high CPU usage in-case fq_codel goes into drop mode.
The problem is that drop mode was considered rare, and implementation wise
it was chosen to be more expensive (to save cycles on normal mode).
Unfortunately is it easy to trigger with an UDP flood. Drop mode is
especially expensive for smaller devices, as it scans a 4K big array, thus
64 cache misses for small devices!
The fix is to allow drop-mode to bulk-drop more packets when entering
drop-mode (default 64 bulk drop). That way we don't suddenly experience a
significantly higher processing cost per packet, but instead can amortize
this.
This is basically the conclusion of a very long email thread avail via:
http://thread.gmane.org/gmane.network.routing.codel/873
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Ticket URL: <https://dev.openwrt.org/ticket/22349>
OpenWrt <http://openwrt.org>
Opensource Wireless Router Technology
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