On 2/15/2019 11:18 AM, Cong Wang wrote:
On Thu, Feb 14, 2019 at 4:43 PM Samudrala, Sridhar
<sridhar.samudr...@intel.com> wrote:
On 2/14/2019 12:15 PM, Cong Wang wrote:
Hello,
While looking into the busy polling in Linux kernel, three questions
come into my mind:
1. In the document[1], it claims sysctl.net.busy_poll depends on
either SO_BUSY_POLL or sysctl.net.busy_read. However, from the code in
ep_set_busy_poll_napi_id(), I don't see such a dependency. It simply
checks sysctl_net_busy_poll and sk->sk_napi_id, but sk->sk_napi_id is
always set as long as we enable CONFIG_NET_RX_BUSY_POLL. So what I am
missing here?
epoll based busypoll is only based on global sysctl_net_busy_poll.
busy_poll value is used with poll()/select()/epoll and and busy_read is used
with socket recvmsg
Right, I was confused by what the term "poll" refers to.
2. Why there is no socket option for sysctl.net.busy_poll? Clearly
sysctl_net_busy_poll is global and SO_BUSY_POLL only works for
sysctl.net.busy_read.
Not sure if it is useful to make it a per socket option. I think it could
be a per poll/epoll fd option
It is useful, however, from Willem's reply it looks like hard to push it
down from epoll() interface to each socket.
3. How is SO_INCOMING_NAPI_ID supposed to be used? I can't find any
useful documents online. Any example or more detailed doc?
A app can create one worker thread per device queue and a worker thread
for an incoming connection can be selected based on SO_INCOMING_NAPI_ID so that
all connections coming on a queue are processed by the same thread. This will
allow epoll from a thread to be associated with sockets that receive packets
from a single queue allowing busy polling.
This information is very useful. It also requires each thread pinning to each
CPU/RX queue, right?
Thread pinning will not be required if XPS is setup to use xps-rxqs that
allows
configuring symmetric queues so that tx-queue is selected based on rx-queue.
See XPS using receive queues map in Documentation/networking/scaling.txt
Anyway, I will add this information to socket.7 man page.
Thanks!
