linux-kernel - Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable

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Date:	Tue, 2 Sep 2014 12:24:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Wang <jasowang@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is
 runnable

On Tue, Sep 02, 2014 at 12:03:42PM +0800, Jason Wang wrote:
> On 09/01/2014 06:19 PM, Peter Zijlstra wrote:
> > OK I suppose that more or less makes sense, the contextual behaviour is
> > of course tedious in that it makes behaviour less predictable. The
> > 'other' tasks might not want to generate data and you then destroy
> > throughput by not spinning.
> 
> The patch try to make sure:
> - the the performance of busy read was not worse than it was disabled in
> any cases.
> - the performance improvement of a single socket was not achieved by
> sacrificing the total performance (all other processes) of the system
>  
> If 'other' tasks are also CPU or I/O intensive jobs, we switch to do
> them so the total performance were kept or even increased, and the
> performance of current process were guaranteed not worse than when busy
> read was disabled (or even better since it may still do busy read
> sometimes when it was the only runnable process). If 'other' task are
> not intensive, they just do little work and sleep soon, then the busy
> read can still work in most of the time during the future reads, we may
> still get obvious improvements.

Not entirely true; the select/poll whatever will now block, which means
we need a wakeup, which increases the latency immensely.

> > I'm not entirely sure I see how its all supposed to work though; the
> > various poll functions call sk_busy_poll() and do_select() also loops.
> >
> > The patch only kills the sk_busy_poll() loop, but then do_select() will
> > still loop and not sleep, so how is this helping?
> 
> Yes, the patch only help for processes who did a blocking reads (busy
> read). For select(), maybe we can do the same thing but need more test
> and thoughts.

What's the blocking read callgraph, how so we end up in sk_busy_poll() there?

But that's another reason the patch is wrong.
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