Re: [9fans] fs performance

[David Leimbach <leimy2k@gmail.com>]

On Sunday, January 9, 2011, ron minnich <rminnich@gmail.com> wrote:
> On Sun, Jan 9, 2011 at 1:38 PM, Bakul Shah <bakul+plan9@bitblocks.com> wrote:
>
>> I didn't say plan9 "suffers". Merely that one has to look at
>> other aspects as well (implying putting in Tstream may not
>> make a huge difference).
>
> well, what we do know from one set of measurements is that it makes a
> measurable difference when latency is measured in the tens of
> milliseconds. :-)
>
> I have done some of these other measurements, e.g. system call
> overhead. Plan 9 system call time is quite a bit longer than Linux
> nowadays, when Linux uses the SYSENTER support.

Linux maps the kernel in the high 1GB of VM too doesn't it?  What does
Plan 9 do (haven't looked yet)

>
> At the same time, the Plan 9 "mon" device that Andrey wrote was
> considerably faster than the procfs-based "mon" device I wrote: 30K
> samples/second on Plan 9 vs. 12K samples/second on Linux.
>
> John did do some measurement of file system times via the trace device
> we wrote. I think it's fair to say that the IO path for fossil is
> considerably slower than the IO path for kernel-based file systems in
> Linux: slower as in multiples of 10, not multiples. There's a fair
> amount of copying, allocation, and bouncing in and out of the kernel,
> and this activity does not come cheap.
>
> So, one speculation is that a kernel-based Plan 9 file system might be
> quite fast. And that's enough random text for a Sunday.

Well number of syscalls to hand off delegating filesystem tasks to a
userspace filesystem implementation is key.  Microkernels try to
optimize this as do virtualization hypervisors, because, as observed,
bouncing around between kernel and userspace gives performance hopes
the beat-down.


>
> ron
>
>