linux-kernel - Re: [PATCH 9/11] Panic delay fix

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Message-ID: <45D36B49.5090109@vmware.com>
Date:	Wed, 14 Feb 2007 12:04:25 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Alan <alan@...rguk.ukuu.org.uk>
CC:	Rusty Russell <rusty@...tcorp.com.au>, Pavel Machek <pavel@....cz>,
	Andi Kleen <ak@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH 9/11] Panic delay fix

Alan wrote:
>> We'd have to audit and figure out what udelays are for hardware and
>> which are not, but the evidence is that the vast majority of them are
>> for hardware and not needed for virtualization.
>>     
>
> Which is irrelevant since the hardware drivers won't be used in a
> virtualised environment with any kind of performance optimisation.
>   

Which is why an audit is irrelevant for the most part.  Note on the 
performance below.

>> Changing udelay to "hardware_udelay" or something all over the kernel
>> would have delayed the paravirt_ops merge by an infinite amount 8)
>>     
>
> paravirt_ops has no business fiddling with udelay. Not only does it
> create more code bloat and stalls in relatively fast paths but its
> optimising the wrong thing anyway.
>   

???  I fail to see the code bloat and also the fast paths.  Which fast 
paths use udelay?

> My performance sucks -> optimise out udelay is the wrong approach. My
> performance sucks, switch to the virtual block driver is the right
> approach, and a virtual block driver won't be using udelay anyway
>   

This is not to stop performance from sucking.  It doesn't.  This is not 
an "approach".  Sure, a virtual block driver won't be using udelay.  
Everyone else who writes hypervisors writes virtual block drivers 
because they don't have optimized I/O emulation for real hardware.  
Their performance sucks without it because they have to go switch to 
some other context and run a device emulator.  Our doesn't.  We have 
optimized almost every I/O device we emulate.  But sitting around 
spinning in udelay is wasting everybody's time.  There is an overhead 
cost to trapping out on I/O instructions.  Removing the udelays that 
typically happen around I/O instructions causes the emulation to break even.

And that is a good thing.  It's certainly not required, nor is it a 
significant win while the kernel is running.  It does cut the boot time 
by a lot, and you will notice an obvious difference with a much faster 
kernel boot simply because a lot of the hardware setup has very 
conservative udelays which take a lot of time during device 
initialization.  Since boot time * number of reboots has a direct impact 
on the number of 9's you can claim for uptime, this is actually a large 
win for reliability.

Zach
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