From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Keir Fraser <keir@xen.org>, Jan Beulich <jbeulich@suse.com>,
xen-devel@lists.xen.org
Subject: Re: [PATCH 3 of 5] rombios/ata: Reading this status register has no relevant side effects
Date: Mon, 30 Jul 2012 22:57:31 +0100 [thread overview]
Message-ID: <20120730225731.70f6842a@ultron> (raw)
In-Reply-To: <5e8fbfea222946369f08.1343677642@andrewcoop.uk.xensource.com>
On Mon, 30 Jul 2012 20:47:22 +0100
Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> So taking two traps when one will do is pointless. This codepath is on the int
> 0x13 hot path, and removing it has about a 30% reduction in the number of traps
> to Qemu during Win7 boot.
You can't read the status for 400nS after a command issue, so throwing
one away is a typical way to handle that.
All of this is optimising the wrong thing.
The problem is that neither kvm not xen have the most basic prediction
handlers in the kernel side exception code so keep hitting qemu.
For a 99% of the ATA transfers you can predict the next few in and outs
and pre-load them into your trap handler avoiding bouncing into qemu, on
a miss you go back into qemu and load the next prediction block (or tree
even)
That's the kind of optimisation that will really make it fly.
Alan
next prev parent reply other threads:[~2012-07-30 21:57 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-30 19:47 [PATCH 0 of 5] Rombios PIO performance Andrew Cooper
2012-07-30 19:47 ` [PATCH 1 of 5] rombios/keyboard: Don't needlessly poll the status register Andrew Cooper
2012-07-30 19:47 ` [PATCH 2 of 5] rombios/ata: Do not wait for BSY to be set Andrew Cooper
2012-07-30 20:18 ` Pasi Kärkkäinen
2012-07-31 9:37 ` [PATCH 2 of 5] (V2) " Andrew Cooper
2012-07-30 19:47 ` [PATCH 3 of 5] rombios/ata: Reading this status register has no relevant side effects Andrew Cooper
2012-07-30 21:57 ` Alan Cox [this message]
2012-07-31 10:08 ` David Vrabel
2012-07-31 10:28 ` Andrew Cooper
2012-07-30 19:47 ` [PATCH 4 of 5] rombios/ata Remove more needless traps from the int 0x13 path Andrew Cooper
2012-07-30 19:47 ` [PATCH 5 of 5] rombios/debug: Reduce verbosity of rombios Andrew Cooper
-- strict thread matches above, loose matches on Subject: below --
2012-11-26 16:41 [PATCH 0 of 5] HVM performance improvements Andrew Cooper
2012-11-26 16:41 ` [PATCH 3 of 5] rombios/ata: Reading this status register has no relevant side effects Andrew Cooper
2012-11-26 16:51 ` Ian Campbell
2012-11-26 16:56 ` Andrew Cooper
2012-11-26 17:01 ` Ian Campbell
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