linux-kernel - RE: irq load balancing

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Date:	Thu, 13 Sep 2007 14:02:15 -0700
From:	"Venkat Subbiah" <venkats@...nite.com>
To:	"Lennart Sorensen" <lsorense@...lub.uwaterloo.ca>
Cc:	"Chris Snook" <csnook@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: RE: irq load balancing

Since most network devices have a single status register for both
receiver and transmit (and errors and the like), which needs a lock to
protect access, you will likely end up with serious thrashing of moving
the lock between cpus.
> Any ways to measure the trashing of locks?

Since most network devices have a single status register for both
receiver and transmit (and errors and the like)
> These register accesses will be mostly within the irq handler which I
plan on keeping on the same processor. The network driver is actually
tg3. Will looks closely into the driver.

Thx,
Venkat


-----Original Message-----
From: Lennart Sorensen [mailto:lsorense@...lub.uwaterloo.ca] 
Sent: Thursday, September 13, 2007 1:45 PM
To: Venkat Subbiah
Cc: Chris Snook; linux-kernel@...r.kernel.org
Subject: Re: irq load balancing

On Thu, Sep 13, 2007 at 01:31:39PM -0700, Venkat Subbiah wrote:
> Doing it in a round-robin fashion will be disastrous for performance.
> Your cache miss rate will go through the roof and you'll hit the slow
> paths in the network stack most of the time.
> > Most of the work in my system is spent in enrypt/decrypting traffic.
> Right now all this is done in a tasklet within the softirqd and hence
> all landing up on the same CPU.
> On the receive side it'a packet handler that handles the traffic. On
the
> tx side it's done within the transmit path of the packet. So would
> re-architecting this to move the rx packet handler to a different
kernel
> thread(with smp affinity to one CPU) and tx to a different kernel
> thread(with SMP affinity to a different CPU) be advisable. 
> What's the impact on cache miss and slowpath/fastpath in network
stack.

Since most network devices have a single status register for both
receiver and transmit (and errors and the like), which needs a lock to
protect access, you will likely end up with serious thrashing of moving
the lock between cpus.

--
Len Sorensen
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