Re: [PATCH] Avoiding fragmentation through different allocator

[Andi Kleen]

On Tue, Jan 25, 2005 at 09:02:34AM -0500, Mukker, Atul wrote:
>  
> > e.g. performance on megaraid controllers (very popular 
> > because a big PC vendor ships them) was always quite bad on 
> > Linux. Up to the point that specific IO workloads run half as 
> > fast on a megaraid compared to other controllers. I heard 
> > they do work better on Windows.
> > 
> <snip>
> > Ideally the Linux IO patterns would look similar to the 
> > Windows IO patterns, then we could reuse all the 
> > optimizations the controller vendors did for Windows :)
> 
> LSI would leave no stone unturned to make the performance better for
> megaraid controllers under Linux. If you have some hard data in relation to
> comparison of performance for adapters from other vendors, please share with
> us. We would definitely strive to better it.

Sorry for being vague on this. I don't have much hard data on this,
just telling an annecdote. The issue we saw was over a year ago
and on a machine running an IO intensive multi process stress test
(I believe it was an AIM7 variant with some tweaked workfile). When the test
was moved to a machine with megaraid controller it ran significantly
lower, compared to the old setup with a non RAID SCSI controller from
a different vendor. I unfortunately don't know anymore the exact
type/firmware revision etc. of the megaraid that showed the problem.

If you have already fixed the issues then please accept my apologies.

> The megaraid driver is open source, do you see anything that driver can do
> to improve performance. We would greatly appreciate any feedback in this
> regard and definitely incorporate in the driver. The FW under Linux and
> windows is same, so I do not see how the megaraid stack should perform
> differently under Linux and windows?

My understanding (may be incomplete) of the issue is basically what
Steve said: something in the stack doesn't like the Linux IO patterns
with often relatively long SG lists, which are longer than in some
other popular OS. This is unlikely to be the Linux driver
(drivers tend to just pass the SG lists through without too much processing),
more likely it was the firmware or something below.

-Andi
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