netdev - Re: [PATCH v10 net-next 0/6] net: low latency Ethernet device polling

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Date:	Tue, 11 Jun 2013 00:32:44 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, jesse.brandeburg@...el.com,
	donald.c.skidmore@...el.com, e1000-devel@...ts.sourceforge.net,
	willemb@...gle.com, bhutchings@...arflare.com, andi@...stfloor.org,
	hpa@...or.com, eilong@...adcom.com, or.gerlitz@...il.com,
	amirv@...lanox.com, eliezer@...ir.org.il
Subject: Re: [PATCH v10 net-next 0/6] net: low latency Ethernet device
 polling

On Tue, 2013-06-11 at 09:49 +0300, Eliezer Tamir wrote:

> I would like to hear opinions on what needs to be added to make this
> feature complete.
> 
> The list I have so far is:
> 1. add a socket option

Yes, please. I do not believe all sockets on the machine are candidate
for low latency. In fact very few of them should be, depending on the
number of cpu and/or RX queues.

> 2. support for poll/select

As long as the cost of llpoll is bounded per poll()/select() call it
will be ok.

> 3. support for epoll

For this one, I honestly do not know how to proceed.

epoll Edge Trigger model is driven by the wakeups events.

The wakeups come from frames being delivered by the NIC (for UDP/TCP
sockets)

If epoll_wait() has to scan the list of epitem to be able to perform the
llpoll callback, it will be too slow : We come back to poll() model,
with O(N) execution time.

Ideally we would have to callback llpoll not from the tcp_poll(), but
right before putting current thread in wait mode.

> 
> Also, would you accept a trailing whitespace cleanup patch for
> fs/select.c?

This has to be submitted to lkml

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