Re: Poor performance during disk writes

["Dieter Nützel" <Dieter.Nuetzel@hamburg.de>]

On Thursday, 20.12.201, 10:49 Helge Hafting wrote:
> jlm wrote:
[-]
> > So I guess I don't really care what mode the hard drive is operating in
> > (udma, mdma, dma or plain ide), I just don't want to have to go get a
> > cup of coffee while the hard drive saves some data. Is there a "don't
>
> Devices generally get the cpu before anything else.  A good disk system
> don't need much cpu.  Running IDE in PIO mode require a lot
> of cpu though.   Using any of the DMA modes avoids that.

Amen..
Sorry, Helge sure you are right in theory but try dbench 32 (maybe 
bonnie/bonnie++) and playing an MP3/Ogg-Vorbis in parallel...
That's my first test on any "new" kernel version.

Even with an 1 GHz Athlon II, 640 MB, U160 DDYS 18 GB, 10k IBM disk (on an 
AHA-2940UW) it stutters like mad. I am running all my kernel _with_ Robert 
Love's preempt + lock-break patches and it doesn't solve the problem.
CPU load is (very) low but it do not work like it should.

> > pre-empt the rest of the system" switch for the eide drives? Is there
> > something fundamental/unique going on here that I'm missing?
> dma, udma, etc. is that switch.  It lets the cpu do other work (such as
> redrawing X) while the disk is busy.  Plain ide is what you don't want.

See above the whole system show some bad hiccup.

> The problem of waiting for other files or swapping while a really big
> write is going on is different.  Get more drives, so the big writes go
> to one drive while you get stuff swapped in (or other file access)
> on other drive(s).  The kernel is capable of getting fast response
> from one drive while another is completely bogged down with
> enormous writes.

Tried this already. Neither I put my test files (MP3/Ogg-Vorbis) in /dev/shm 
or a nother disk it do not change anything.

There must be something in the VFS?

-Dieter

-- 
Dieter Nützel
Graduate Student, Computer Science
University of Hamburg
@home: Dieter.Nuetzel@hamburg.de