Sorry for the delay...
On Thu, 29 Dec 2005, Ingo Molnar wrote:
>
> * Nicolas Pitre <[email protected]> wrote:
>
> > This is with all mutex patches applied and CONFIG_DEBUG_MUTEX_FULL=n,
> > therefore using the current semaphore code:
> >
> > text data bss dec hex filename
> > 1821108 287792 88264 2197164 2186ac vmlinux
> >
> > Now with CONFIG_DEBUG_MUTEX_FULL=y to substitute semaphores with
> > mutexes:
> >
> > text data bss dec hex filename
> > 1797108 287568 88172 2172848 2127b0 vmlinux
> >
> > Finally with CONFIG_DEBUG_MUTEX_FULL=y and fast paths inlined:
> >
> > text data bss dec hex filename
> > 1807824 287136 88172 2183132 214fdc vmlinux
> >
> > This last case is not the smallest, but it is the fastest.
>
> i.e. 1.3% text savings from going to mutexes, and inlining them again
> gives up 0.5% of that. We've uninlined stuff for a smaller gain in the
> past ...
>
> > > Note that x86 went to a non-inlined fastpath _despite_
> > > having a compact CISC semaphore fastpath.
> >
> > The function call overhead on x86 is less significant than the ARM
> > one, so always calling out of line code might be sensible in that
> > case.
>
> i'm highly doubtful we should do that. The spinlock APIs are 4 times
> more frequent than mutexes are ever going to be, still they too are
> mostly out of line. (and we only inline the unlock portions that are a
> space win!) Can you measure any significant difference in performance?
> (e.g. lat_pipe triggers the mutex fastpath, in DEBUG_MUTEX_FULL=y mode)
Here it is.
With the default non inlined fast path:
Pipe latency: 14.2669 microseconds
Pipe latency: 14.2625 microseconds
Pipe latency: 14.2655 microseconds
Pipe latency: 14.2670 microseconds
Then, with fast paths inlined:
Pipe latency: 13.9483 microseconds
Pipe latency: 13.9409 microseconds
Pipe latency: 13.9468 microseconds
Pipe latency: 13.9529 microseconds
So inlining the mutex fast path is more than 2% faster for the whole
test.
Is that worth the 0.5% increase in kernel size in your opinion?
Note: I modified lat_pipe to use two threads instead of two processes
since on ARM switching mm require a full cache flush due to the VIVT
cache, and doing so skyrockets the pipe latency up to 480 microsecs with
the mutex difference lost in the noise.
Nicolas
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
