Re: [linux-audio-dev] Re: [announce] [patch] Voluntary Kernel Preemption Patch

[Lee Revell <rlrevell@joe-job.com>]

On Mon, 2004-07-19 at 06:48, Ingo Molnar wrote:
> * Lee Revell <rlrevell@joe-job.com> wrote:
> 
> > Just as a reference point, what do you think is the longest delay I
> > *should* be seeing?  I recall hearing that BEOS guaranteed that
> > interrupts are never disabled for more than 50 usecs.  This seems
> > achievable, as the average delay I am seeing is 15 usecs.
> 
> ATA hardirq latency can be as high as 700 usecs under load even on
> modern hw, when big DMA requests are created with long scatter-gather
> lists. We also moved some of the page IO completion code into irq
> context which further increased hardirq latencies. Since these all touch
> cold cachelines it all adds up quite quickly.
> 

Does this scale in a linear fashion with respect to CPU speed?  You
mentioned you were testing on a 2Ghz machine, does 700 usecs on that
translate to 2800 usecs on a 500Mhz box?

On a 2Ghz machine, 700 usecs is about one million CPU cycles.  In other
words, the highest priority process can become runnable, then have to
wait *one million cycles* to get the CPU.

How much I/O do you allow to be in flight at once?  It seems like by
decreasing the maximum size of I/O that you handle in one interrupt you
could improve this quite a bit.  Disk throughput is good enough, anyone
in the real world who would feel a 10% hit would just throw hardware at
the problem.

Lee