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Re: Linux users want better desktop performance (Screw data. Prioritize code)

Mattias Hellström wrote:
On Tue, Feb 17, 2009 at 8:19 PM, Valent Turkovic
<valent turkovic gmail com> wrote:

What is you comment?

Do not blame the operating system for badly coded apps. Only the app
knows if caching is a good idea.

Exerpt from "man -s 2 open"
O_DIRECT (Since Linux 2.4.10)
              Try  to  minimize cache effects of the I/O to and from
this file.  In general this will degrade perfor-
              mance, but it is useful in special situations, such as
when applications do their  own  caching.   File
              I/O is done directly to/from user space buffers.  The
I/O is synchronous, that is, at the completion of
              a read(2) or write(2), data is guaranteed to have been
transferred.  See NOTES below for  further  dis-

Just let me add that DIRECT will usually slightly slow the i/o it's doing, but greatly reduce the impact on the system. This can really save performance of machines doing small transactions such as serving DNS, DHCP, mail, NNTP, of database lookups. The transaction per sec of the application can drop by 60% writing a large file, if you can pipe it through dd with the direct option it will hurt less.

So if the file i/o is non-critical there is a big gain to be had for the performance of some applications.

Bill Davidsen <davidsen tmr com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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