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SubjectLow latency patch for IDE (kernel 2.2.14)

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:

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.


Giuseppe Guerrini (HOME) (WORK)

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