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Re: [Qemu-devel] Re: 2.6.24 says "serial8250: too much work for irq4" a

From: H. Peter Anvin
Subject: Re: [Qemu-devel] Re: 2.6.24 says "serial8250: too much work for irq4" a lot.
Date: Fri, 08 Feb 2008 23:12:12 -0800
User-agent: Thunderbird (X11/20071115)

Blue Swirl wrote:
On 2/9/08, Rob Landley <address@hidden> wrote:
Here's a patch Peter Anvin wrote so the serial I/O doesn't flood the kernel.

The patch looks OK, but the throttling should benefit all devices, as
discussed here:

I strongly disagree with the sentiments in that post.

This is not a matter of rate throttling, but simulated FIFO exhaustion -- they are NOT the same thing. Simulated FIFO exhaustion is functionally equivalent to making sure there are interrupt windows opened in an otherwise-too-long critical section; it doesn't constrain any particular flow rate, as it still permits another interrupt to immediately come in.

If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns.


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