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Re: [Qemu-devel] interrupt handling in qemu

From: Xin Tong
Subject: Re: [Qemu-devel] interrupt handling in qemu
Date: Tue, 27 Dec 2011 18:23:33 -0700

On Tue, Dec 27, 2011 at 6:10 PM, Peter Maydell <address@hidden> wrote:
> On 28 December 2011 00:43, Xin Tong <address@hidden> wrote:
>> Which version of QEMU did you do your test on, and what are the tests.
> It was whatever trunk qemu was a year or so ago, and the test was
> just "time to login prompt for ARM guest in system mode".
>> I modified QEMU to check for interrupt status at the end of every TB
>> and ran it on SPECINT2000 benchmarks with QEMU 0.15.0. The performance
>> is 70% of the unmodified one for some benchmarks on a x86_64 host.
> I don't suppose you could provide a brief set of instructions for
> setting up a benchmark setup like this? (Is this user-mode or
> system-mode?)

It is in system mode, the SPECINT2000 benchmarks are running on top of
a ubuntu linux. It is not trial to set up.

>> I
>> agree that the extra load-test-branch-not-taken per TB is minimal, but
>> what I found is that the average number of TB executed per TB enter is
>> low (~3.5 TBs), while the unmodified approach has ~10 TBs per TB
>> enter. this makes me wonder why. Maybe the mechanism i used to gather
>> this statistics is flawed. but the performance is indeed hindered.
> Odd.
>> By keeping a counter that decrements on every TB, and when the counter
>> reaches 0, the current executing TB checks for interrupt status.
> Decrementing a counter on every TB is going to be slower than
> just checking a flag, so you might as well just check the flag.
> (If the flag is set you need to handle the interrupt anyway
> so there's no point delaying it.)
The point I am trying to make here is that if qemu TB exits for every
interrupt, it is going to be too many. What if QEMU exits and handle a
few interrupts in one exit.
> -- PMM

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