Re: Fwd: Adaptive read-ahead V12

Wu Fengguang wrote:
----- Forwarded message from Iozone <capps@xxxxxxxxxx> -----

Subject: Adaptive read-ahead V12
From: Iozone <capps@xxxxxxxxxx>
To: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
X-Mailer: Microsoft Outlook Express 6.00.2900.2670
Date: Thu, 25 May 2006 11:44:37 -0500

Wu Fengguang,

I see that Andrew M. is giving you some pushback.... His argument is that the application could do a better job
of scheduling its own read-ahead. ( I've heard this one before)

My thoughts on this argument would be along the lines of:

Indeed the application might be able to do a better
job, however expecting, or demanding, the rewrite
of all applications to behave better might be an unreasonable
expectation.

A reasonable expectation would be that the application would have a way to tell the kernel to ignore readahead for a given file, other than changing the behavior of the kernel as a whole for all processes on the machine. This smart application could then use aio or some other similar method to do preread itself.

My personal opinion is that the kernel only does a good job reading ahead for sequential access, and since that's the common case only a means of preventing that effort need be provided.

I can think of several ways the kernel might be smarter about the way it reads (or not) random access, but since I have no way to test them on applications other than my own I won't belabor them here.

--
Bill Davidsen <davidsen@xxxxxxx>
Obscure bug of 2004: BASH BUFFER OVERFLOW - if bash is being run by a
normal user and is setuid root, with the "vi" line edit mode selected,
and the character set is "big5," an off-by-one errors occurs during
wildcard (glob) expansion.

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