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fukawi2

Ex-Administratorino

From: .vic.au

Registered: 2007-09-28

Posts: 6,223

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Why Deadline Scheduler for JFS

According to the Wiki, using the Deadline Scheduler is supposed to improve the performance of JFS (http://wiki.archlinux.org/index.php/JFS … _Scheduler)

Knowing not much about I/O schedulers, I went in pursuit of information... Reading the article below, I can't quite understand how Deadline would improve performance of the Anticipatory Scheduler?
http://www.linuxjournal.com/article/6931

This didn't shed much light that wasn't in the first article either:
http://www.wlug.org.nz/LinuxIoScheduler

Does anyone have an explanation for why Deadline would improve performance on JFS? Why would the 6ms pause while waiting for more read requests outbalance the extra seek times?

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theringmaster

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From: Air Force

Registered: 2007-07-16

Posts: 581

Website

Re: Why Deadline Scheduler for JFS

http://en.wikipedia.org/wiki/Deadline_scheduler

has a nice bit of info in it.

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fukawi2

Ex-Administratorino

From: .vic.au

Registered: 2007-09-28

Posts: 6,223

Website

Re: Why Deadline Scheduler for JFS

The first article I linked to explained it all pretty well; I'm understanding the differences between the schedulers, but I'm not understanding how JFS is making the DS better than the AS...

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BJJ

Member

Registered: 2007-10-23

Posts: 25

Re: Why Deadline Scheduler for JFS

http://en.wikipedia.org/wiki/JFS_(file_system)

It looks like JFS already has a own form of 'anticipation'.
See the sections Concurrent Input / Output (CIO) and Allocation Groups.

Another anticipator like AS would only increase overhead, therefore deadline would work beter.

jleach

Member

Registered: 2008-07-24

Posts: 27

Re: Why Deadline Scheduler for JFS

One of the shortcomings of the anticipatory scheduler is that it assumes only 1 physical reading head, only 1 read request is dispatched to the disk controller at a time, and read anticipation assumes synchronous requests are issued by individual processes.

This makes it weaker for RAID systems, which I assume is generally where the talk of "deadline is superior for filesystem X" comes up.

In truth CFQ and deadline have similar results.  Beyond that one would be comparing filesystems, and though I use and like JFS, it's not regarded as the fastest.

Other info:
http://www.cs.ccu.edu.tw/~lhr89/linux-k … dulers.pdf
http://edgyu.excess.org/ols/2004/Steven … dulers.pdf

fukawi2

Ex-Administratorino

From: .vic.au

Registered: 2007-09-28

Posts: 6,223

Website

Re: Why Deadline Scheduler for JFS

Thanks guys
There's some good info in that, cheers

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