Re: [announce] [patch] limiting IRQ load, irq-rewrite-2.4.11-B5

Benjamin LaHaise
2001-10-02T01:04:56Z

On Mon, Oct 01, 2001 at 08:41:20PM -0400, jamal wrote:
>
> >The new mechanizm:
> >
> >- the irq handling code has been extended to support 'soft mitigation',
> > ie. to mitigate the rate of hardware interrupts, without support from
> > the actual hardware. There is a reasonable default, but the value can
> > also be decreased/increased on a per-irq basis via
> > /proc/irq/NR/max_rate.
>
> I am sorry, but this is bogus. There is no _reasonable value_. Reasonable
> value is dependent on system load and has never been and never
> will be measured by interupt rates. Even in non-work conserving schemes

It is not dependant on system load, but rather on the performance of the
CPU and the number of interrupt sources in the system.

> There is already a feedback system that is built into 2.4 that
> measures system load by the rate at which the system processes the backlog
> queue. Look at netif_rx return values. The only driver that utilizes this
> is currently the tulip. Look at the tulip code.
> This in conjuction with h/ware flow control should give you sustainable
> system.

Not quite. You're still ignoring the effect of interrupts on the users'
ability to execute instructions during their timeslice.

> [Granted that mitigation is a hardware specific solution; the scheme we
> presented at the kernel summit is the next level to this and will be
> non-dependednt on h/ware.]
>
> >(note that in case of shared interrupts, another 'innocent' device might
> >stay disabled for some short amount of time as well - but this is not an
> >issue because this mitigation does not make that device inoperable, it
> >just delays its interrupt by up to 10 msecs. Plus, modern systems have
> >properly distributed interrupts.)
>
> This is a _really bad_ idea. not just because you are punishing other
> devices.

I'm afraid I have to disagree with you on this statement. What I will
agree with is that 10msec is too much.

> Lets take network devices as examples: we dont want to disable interupts;
> we want to disable offending actions within the device. For example, it is
> ok to disable/mitigate receive interupts because they are overloading the
> system but not transmit completion because that will add to the overall
> latency.

Wrong. Let me introduce you to my 486DX/33. It has PCI. I'm putting my
gige card into the poor beast. transmitting full out, it can receive a
sufficiently high number of tx done interrupts that it has no CPU cycles left
to run, say, gated in userspace.

Falling back to polled operation is a well known technique in realtime and
reliable systems. By limiting the interrupt rate to a known safe limit,
the system will remain responsive to non-interrupt tasks even under heavy
interrupt loads. This is the point at which a thruput graph on a slow
machine shows a complete breakdown in performance, which is always possible
on a slow enough CPU with a high performance device that takes input from
a remotely controlled user. This is *required*, and is not optional, and
there is no way that a system can avoid it without making every interrupt
a task, but that's a mess nobody wants to see in Linux.

-ben