On 29/07/15 18:07, David Lang wrote:
On Wed, 29 Jul 2015, Alan Jenkins wrote:
On 29/07/15 12:24, Alan Jenkins wrote:
On 29/07/15 05:32, Rosen Penev wrote:
Anyone know what the situation is with kirkwood and BQL? I found a
patch for it but have no idea if there are any issues.
I have such a system but have no idea how to ascertain the efficacy
of BQL.
To the latter:
BQL works for transmissions that reach the full line rate (e.g. for
1000MB ethernet). It limits the queue that builds in the
driver/device to the minimum they need. Then queue mostly builds in
the generic networking stack, where it can be managed effectively
e.g. by fq_codel.
So a simple efficacy test is to run a transmission at full speed,
and monitor latency (ping) at the same time. Just make sure the
device qdisc is set to fq_codel. fq_codel effectively prioritizes
ping, so the difference will be very easy to see.
I don't know if there's any corner cases that want testing as well.
BQL adjusts the number of packets that can be queued based on their
size, so you can have far more 64 byte packets queued than you can
have 1500 byte packets.
do a ping flood of your network with different packet sizes and look
at the queue lengths that are allowed, the queue length should be much
higher with small packets.
BQL can be disabled at runtime for comparison testing:
http://lists.openwall.net/netdev/2011/12/01/112
There's a BQL tool to see it working graphically (using readouts
from the same sysfs directory):
https://github.com/ffainelli/bqlmon
My Kirkwood setup at home is weak, I basically never reach full link
speed. So this might be somewhat academic unless you set the link
speed to 100 or 10 using the ethtool command. (It seems like a good
idea to test those speeds even if you can do better though). You
probably also want to start with offloads (tso, gso, gro) disabled
using ethtool, because they aggregate packets.
a quick test with a 100M setting, connected to gigabit switch, and
flent tcp_download, shows ping under load increases to about 8ms.
Conclusion: the Debian kirkwood kernel probably isn't doing BQL for
me :).
8ms of latency under load is doing very well. what are you expecting?
David Lang
Not to sound like half-way across adsl? I'm describing a LAN test at 100M.
Going against an x86 desktop on gigabit gives 1.2ms. That's more like
what I expected. But there's no bql driver, boo r8169, and 100M gives
about 4ms. Killing offloads doesn't seem to help.
On the laptop end I do have bql. On 100M it goes from about 6ms (with
bql disabled) to 2ms. After remembering to disable offloads, bql takes
it from 1.2ms to 0.7ms.
That final figure sounds good to me. I kind of expected a more dramatic
difference with BQL though.
I also found a simple check for bql (without looking at source code).
The sysfs directories will be present regardless
/sys/class/net/eth0/queues/tx-0/byte_queue_limits/
but if the "limit" file contains "0" even after a test, there's no BQL.
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