Re: coping with memory limitations and packet flooding in codel and fq_codel
From: Eric Dumazet <hidden>
Date: 2012-08-21 08:38:18
On Mon, 2012-08-20 at 14:05 -0700, Dave Taht wrote:
We've had a raft of deployment questions regarding fq_codel, memory use, and the impact of unresponsive flows over on the cerowrt development list. sort of summary of the thread here, with some codel related ideas https://lists.bufferbloat.net/pipermail/cerowrt-devel/2012-August/000423.html some early data on wifi interactions here https://lists.bufferbloat.net/pipermail/cerowrt-devel/2012-August/000407.html
CC netdev
codel / fq_codel have a packet limit per qdisc, like any other linux
qdisc.
DEFAULT_CODEL_LIMIT = 1000 packets
fq_codel : 10240 packets
For memory constrained boxes, it would be wise to add a 'truesize' limit
Not a bytes limit based on skb->len sum, because a skb->len = 60 bytes
packet might consume far more memory (more than 2 Kbytes)
ath9k driver is known to deliver very fat skbs (skb->truesize is too
big)
One 'other way' to limit bloat would be to reallocate skbs when one
qdisc begins to use more than 25 % of its truesize limit
enqueue(q, skb)
{
if (q->truesize_sum > q->truesize_limit / 4)
skb = skb_reduce_truesize(skb, GFP_ATOMIC);
...
}
This is what some drivers already do in their rx handler (this is called
copybreak), but this would do the copy (and extra cpu cost) only in
stress situations.
Note : we also have a very big struct skb_shared_info (320 bytes), we
probably could shrink it for packets without fragments.