Re: [linux-audio-dev] Re: [DATAPOINT] kernels and latencies

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Subject: Re: [linux-audio-dev] Re: [DATAPOINT] kernels and latencies
From: Roger Larsson (roger.larsson_AT_norran.net)
Date: Mon Jul 03 2000 - 22:03:34 EEST


Andrew Morton wrote:
>
> Roger Larsson wrote:
> >
> > Hi,
> >
> > I have gone trough my collected data and found that
> > the best yet (non latency patched) is
> > test1-ac22-riel++
> > It works perfectly for streaming read/write/copy
> > (latencies to SCHED_FIFO process below 5 ms !)
> > but fails for mmap002 with latencies > 180 ms
>
> With a conditional reschedule in generic_file_read,
> generic_file_write and truncate_inode_pages the scheduling
> latency falls from 130 millisecs to 4 millisecs during a kernel build.
>

That is good!

> I don't think mmap002 is a very pointful test. "Don't
> run mmap002 when you're doing multimedia".
>

Sadly it is...
There will be other programs that utilizes mmap that can give the same
problems...

 
> Here's the story on mmap002:
>
> switchin_0144:0 -> switchout_0144:0 2600 .90 159336.00 319.49 830693.16
> start_0144:0 -> switchout_0144:0 2 2552.31 100519.87 51536.09 103072.18
> open.c:811 -> open.c:813 78 .15 74553.96 960.08 74886.74
> switchin_0120:0 -> switchout_0120:0 2418 .65 58210.29 451.25 1091147.90
> switchin_0001:0 -> open.c:806 1 50005.77 50005.77 50005.77 50005.77
> start_0001:0 -> switchout_0001:0 1 14107.08 14107.08 14107.08 14107.08
> switchin_0001:0 -> switchout_0001:0 54 263.45 3880.48 1696.84 91629.60
> switchin_0106:0 -> stop_0106:0 4 3.50 1420.02 419.48 1677.94
> start_0011:0 -> open.c:834 3 220.58 719.98 487.38 1462.15
>
> ^^^^^^^^^
> This column
>
> 160 milliseconds in sys_msync() -> "don't use sys_msync()...."

What do you use to get this output?

>
> That 75 millisecs in open.c is the fput(filp) in sys_close(). Investigating...
>
> syscall 120 is sys_clone.
>
> syscall 1 is sys_exit.
>
> All very interesting, but really the only thing which needs tweaking in
> the filesystem apart from the patch below appears to be sys_close(). After that
> I'll take a look at X11 and /proc performance.
>
> On a fast uniprocessor, it is going to be very hard to do better
> than 3.5 milliseconds.
>

That will be good enough...

Note: I have seen X causing 50 ms latencies...
[aux_write_dev+26/28]

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