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Re: realtime section bugs still around

To: Jason Newton <nevion@xxxxxxxxx>
Subject: Re: realtime section bugs still around
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Tue, 31 Jul 2012 22:55:51 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <CAGou9MhneejOuhX4c8G06c3Zh7dxF-OtZ+=mT-7fho_u1Q3zWw@xxxxxxxxxxxxxx>
References: <CAGou9MgezsS=2+SngGWBJv5Npsuqacx1VPJwvMuf0FS+XnXt8A@xxxxxxxxxxxxxx> <20120730030333.GE2877@dastard> <CAGou9MheeBWxajd65szNfDB2L+VVoZ7SypEdUKj7np3L0H8fHA@xxxxxxxxxxxxxx> <CAGou9MgMHbTPebFUDXdyLvrUr4T9wo-geNtaPUft9uUmOaju6Q@xxxxxxxxxxxxxx> <50186E51.1020107@xxxxxxxxxxxxxxxxx> <CAGou9MhneejOuhX4c8G06c3Zh7dxF-OtZ+=mT-7fho_u1Q3zWw@xxxxxxxxxxxxxx>
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On 7/31/2012 6:55 PM, Jason Newton wrote:
> On Tue, Jul 31, 2012 at 4:46 PM, Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>wrote:
>> On 7/31/2012 6:01 PM, Jason Newton wrote:
>>> I'm still interested in finding out why streaming a few hundred MB to
>> disk
>>> has so much over head in comparison to the calculations I do in
>> userspace,
>> 1.  md eats a lot of cycles at high data rates
> md with intel's raid0?  I stopped using linux/softraid, but  I've read
> intel's is a mix between hardware and software raid...

Intel Matrix RAID is fakeraid.  Designed for consumer workloads.  You're
shoving a decidedly non consumer, high b/w IO stream through it.  Don't
expect much.  In fact I'm surprised you're using consumer grade gear for
this application.  You are designing this software/system for a
commercial use case, correct?  If so I'd get some better hardware.

CPU overhead for fakeraid will be similar to md/RAID, depending on the
vendor and implementation.  In some cases it may be much higher than md.

> 2.  ATA overhead
>> 3.  IRQ/MSI overhead
>> 4.  Etc.
>> All these small bits add up to more than negligible CPU overhead at high
>> data rates.
> Regarding the others, how would I go about measuring their overhead...

To what end?


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