Hello.
Not an expert nor kernel developer, etc., just simple sysadmin
here, but will answer anyway since nobody did yet.
I think we want the mq qdisc to distribute the load between cores,
to support very high speed network cards or too slow CPUs.
Also, if net.core.default_qdisc = fq_codel is used, it also has the
mq qdisc and fq_codel childs for each CPU core, so that's the default
behavior in other distros.
I guess being under mq it could impact fq_codel's ability somewhat,
but it's better to support all the hardware spectrum by default,
otherwise we should use cake as default since it can handle up to
10Gbit/s without troubles; we don't, because Debian is a generic
operating system so it should work on embedded devices, laptops,
workstations and big iron servers...
Also, I don't see a problem with statically linking fq_codel if we
are using it as default, since it will probably get loaded anyway...
Thanks for pushing this forward, doing these tests, etc.!!!
Bye.
El 7/1/21 a las 21:12, Noah Meyerhans escribió:
On Thu, Apr 23, 2020 at 03:34:06PM -0700, Matt Taggart wrote:
#890343 was originally opened against systemd asking to install the upstream
systemd sysctl.d/50-default.conf file that sets:
net.core.default_qdisc = fq_codel
As explained in #950701 (and the systemd debian changelog) the debian
systemd maintainers felt that systemd in debian should not be changing
kernel policies (and I agree).
So #890343 was reassigned to linux to consider changing the default.
fq_codel is better in every way than pfifo_fast and I am unaware of any
reason why it would not be a better default. (but don't trust me, ask the
kernel networking experts)
Can we change it?
I strongly agree that we should make this change for the bullseye
release.
I'm looking into provding a patch to implement the switch to fq_codel by
default, but it appears to require something more than just a kernel
config change. I have tried the following with the 5.10 kernel from the
current sid branch:
CONFIG_NET_SCH_FQ_CODEL=m
CONFIG_DEFAULT_FQ_CODEL=y
CONFIG_DEFAULT_NET_SCH="fq_codel"
Then we don't see any change at all to the qdisc in use:
admin@ip-10-0-0-162:~$ grep -i fq_codel /boot/config-$(uname -r)
CONFIG_NET_SCH_FQ_CODEL=m
CONFIG_DEFAULT_FQ_CODEL=y
CONFIG_DEFAULT_NET_SCH="fq_codel"
admin@ip-10-0-0-162:~$ /sbin/sysctl net.core.default_qdisc
net.core.default_qdisc = pfifo_fast
admin@ip-10-0-0-162:~$ tc qdisc show dev ens5
qdisc mq 0: root
qdisc pfifo_fast 0: parent :2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc pfifo_fast 0: parent :1 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
admin@ip-10-0-0-162:~$ ip link show dev ens5
2: ens5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc mq state UP mode
DEFAULT group default qlen 1000
link/ether 02:47:e2:7c:be:ff brd ff:ff:ff:ff:ff:ff
altname enp0s5
If we statically link the fq_codel module into the kernel, then we see:
admin@ip-10-0-0-162:~$ grep -i fq_codel /boot/config-$(uname -r)
CONFIG_NET_SCH_FQ_CODEL=y
CONFIG_DEFAULT_FQ_CODEL=y
CONFIG_DEFAULT_NET_SCH="fq_codel"
admin@ip-10-0-0-162:~$ /sbin/sysctl net.core.default_qdisc
net.core.default_qdisc = fq_codel
admin@ip-10-0-0-162:~$ /sbin/tc qdisc show dev ens5
qdisc mq 0: root
qdisc fq_codel 0: parent :2 limit 10240p flows 1024 quantum 1514 target 5ms
interval 100ms memory_limit 32Mb ecn drop_batch 64
qdisc fq_codel 0: parent :1 limit 10240p flows 1024 quantum 1514 target 5ms
interval 100ms memory_limit 32Mb ecn drop_batch 64
admin@ip-10-0-0-162:~$ ip link show dev ens5
2: ens5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc mq state UP mode
DEFAULT group default qlen 1000
link/ether 02:47:e2:7c:be:ff brd ff:ff:ff:ff:ff:ff
altname enp0s5
So in this case, we have fq_codel configured, but not as the root
qdisc for the interface. If we manually set it:
admin@ip-10-0-0-162:~$ sudo /sbin/tc qdisc add root dev ens5 fq_codel
Then we get the following configuration:
admin@ip-10-0-0-162:~$ /sbin/tc qdisc show dev ens5
qdisc fq_codel 8001: root refcnt 3 limit 10240p flows 1024 quantum 9015 target
5ms interval 100ms memory_limit 32Mb ecn drop_batch 64
admin@ip-10-0-0-162:~$ ip link show dev ens5
2: ens5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc fq_codel state UP
mode DEFAULT group default qlen 1000
link/ether 02:47:e2:7c:be:ff brd ff:ff:ff:ff:ff:ff
altname enp0s5
I believe that this is what we want. Is that accurate?
The recent thread at
https://www.mail-archive.com/netdev@vger.kernel.org/msg380410.html
also seems relevant.
noah
--
Ivan Baldo - iba...@adinet.com.uy - http://ibaldo.codigolibre.net/
Freelance C++/PHP programmer and GNU/Linux systems administrator.
The sky is not the limit!
