Re: [PATCH v3 net-next 0/4] net: low latency Ethernet device polling

From: Alex Rosenbaum
Date: Tue May 21 2013 - 08:08:01 EST

On 5/20/2013 1:15 PM, Eliezer Tamir wrote:
updated with the comments I got so far.

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Hello Eliezer,

I am working in Mellanox on a low latency user space offload technology and there are some similarities between the user space and your kernel implementation.

We have experience in similar âinfinite pollingâ issues in respect to the real applications.

I am coming in a little late here but wanted to check that:

1. It seem this patch does not cover epoll/select and such IO muxing APIs?

Most real application will be based on epoll or select, not like netperf which is a simple send/recv per thread based network test. If you take memcached application you have epoll per thread with few sockets in each running on each core.

In the IO mux cases you need to poll multiple driver rings while also polling other non-network fdâs (files, pipes,..) and not to hurt their latency response.

2. How is the logic aware of RSS and RFS?

With TCP sockets, the driver knows the specific ring it need to poll so this should be mapped and provide the best latency.

For UDP (unicast and multicast) you can have all rings delivering packets to a single receive socket, is ndo_ll_poll expected to scan driver rings?

3. I could not find any reference to multi-thread on single core logic. This can causes the opposite effect and create contentions and higher latencyâs.

Maybe you should ref_count the number of threads per core going into ndo_ll_poll. If the second+ threads want to go down to ndo_ll_poll you should block (sleep) them instead of creating contention.

In this mode at least the first thread will get very good latency and the others will not get hurt.

Or if they move to a different core they should go down to the driver for polling the ring.


Alex Rosenbaum

Director R&D Application Acceleration

Mellanox Technologies

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