Low latency patch for IDE (kernel 2.2.14)

[Giuseppe Guerrini]

 Hi all,

 I'm using (ok, still...) a RH 6.2 system with kernel 2.2.14, with RAID1
enabled. Under heavy disk load, the systen has a latency of up to 700
milliseconds. This harms my "almost-real-time" applications.
 After I unsuccessfully tried all usual system tunings (hdparm...), and
some low-latency patches kernel too, I decided to modify the IDE
driver. The problem was due to an "interrupt burst" that keeps the CPU
busy for a while. My patch defers the IDE interrupt handling by activating
a kernel thread that actually does the job. It works...

 You can find the patch (for kernel 2.2.14) here:

	http://www.geocities.com/giusguerrini/linux/llide-patch.txt

 Note that the kernel thread has a "SCHED_FIFO" scheduling policy, and a
real-time priority of 10. So, only processes of "real-time" class and
priority greater than 10 take advantage of it. Anyway, you can simply
change the policy to "SCHED_OTHER" in function "ide_thread".
 Note also that this is a quite drastic solution, that makes the disk
slower, and the CPU load greater (slightly).

 Comments are appreciated.

	Regards,

	Giuseppe Guerrini

		[e-mail] (HOME)
		[e-mail] (WORK)

 P.S. Please CC to "[e-mail]", because I'm not subscribed
to the list. Thank you.