linux-kernel - Re: Why is NCQ enabled by default by libata? (2.6.20)

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Message-ID: <Pine.LNX.4.64.0703270629180.31853@p34.internal.lan>
Date:	Tue, 27 Mar 2007 06:30:38 -0400 (EDT)
From:	Justin Piszcz <jpiszcz@...idpixels.com>
To:	Tejun Heo <htejun@...il.com>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: Why is NCQ enabled by default by libata? (2.6.20)

On Tue, 27 Mar 2007, Tejun Heo wrote:

> Justin Piszcz wrote:
>> Checking the benchmarks on various hardware websites, anandtech, 
>> hothardware and others, they generally all come to the same conclusion if 
>> there is only 1 thread using I/O (single user system) then NCQ off is the 
>> best.
>
> Are they testing using Linux?  I/O performance is highly dependent on 
> workload and scheduling, so result on windows wouldn't be very useful. 
> Posting some links here would be nice.
>
>> I see 30-50MB/s faster speeds with NCQ turned off on two different SW 
>> RAID5s.
>
> You're testing raptors, right?  If the performance drop is that drastic and 
> consistent over different workloads, we'll have to disable NCQ for raptors. 
> I'm not sure about other drives.  Care to perform tests over more popular 
> ones (e.g. recent seagates or 7200rpm wds)?
>
> -- 
> tejun
>

You are correct, it definitely depends upon the workload, and a lot of the 
benchmarks do use Windows; however, I will have to check later, I recall 
finding a few that did test under Linux.

For a plain untar with lots of small files, the benefit is not as big as 
sequential reads/writes of big files; however, there is still an 
improvement:

Raid5 Quad 150 Raptor (NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'

real    0m21.721s
user    0m0.174s
sys     0m1.541s

Raid5 Quad 150 Raptor (NO NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'

real    0m16.761s
user    0m0.195s
sys     0m1.361s

Raid5 Six 400GB Sata Drives (NO NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'
real    0m54.844s
user    0m0.189s
sys     0m1.432s

Raid5 Six 400GB Sata Drives (NCQ)
# time sh -c 'tar xf linux-2.6.20.tar; sync'
real    1m7.322s
user    0m0.194s
sys     0m1.492s

Justin.
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