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

To: adilger@xxxxxxxxxxxxx (Andreas Dilger)
Subject: Re: [announce] [patch] limiting IRQ load, irq-rewrite-2.4.11-B5
From: kuznet@xxxxxxxxxxxxx
Date: Fri, 5 Oct 2001 23:17:22 +0400 (MSK DST)
Cc: Robert.Olsson@xxxxxxxxxxx, mingo@xxxxxxx, hadi@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, bcrl@xxxxxxxxxx, netdev@xxxxxxxxxxx, torvalds@xxxxxxxxxxxxx, alan@xxxxxxxxxxxxxxxxxxx
In-reply-to: <20011005124824.F315@xxxxxxxxxxxxxx> from "Andreas Dilger" at Oct 5, 1 12:48:24 pm
Sender: owner-netdev@xxxxxxxxxxx
Hello!

> One question which I have is why would you ever want to continue polling
> if there is no work to be done?  Is it a tradeoff between the amount of
> time to handle an IRQ vs. the time to do a poll?

Yes. IRQ even taken alone eat non-trivial amount of resources.

Actually, I remember Jamal worked with machine, which had
no io-apic and only irq ack/mask/unmask eated >15% of cpu there. :-)

>                                                 An assumption that if
> there was previous network traffic there is likely to be more the next
> time the interface is checked (assuming you have other work to do between
> the time you last polled the device and the next poll)?

Exactly.

Note also that the testing of "goto not_done" was made in pure environment:
dedicated router. Continuous polling is an evident advantage in this situation,
only power is eaten. I would not enable this on a notebook. :-)


> Is enabling/disabling of the RX interrupts on the network card an issue
> in the sense of "you need to wait X us after writing to this register
> for it to take effect" or other issue which makes it preferrable to have
> some "hysteresis" between changing state from IRQ-driven to polling?

"some hysteresis" is right word. This loop is an experiment with still
unknown result yet. Originally, Jamal proposed to spin several times.
I killed this. Robert proposed to check inifinite loop yet. (Note,
jiffies check is just a way to get rid of completely idle devices,
one jiffie is enough lonf time to be considered infinite).

Alexey

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