Messages in this thread | | | Date | Fri, 06 Jan 2006 16:20:42 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [patch 2/3] mutex subsystem: fastpath inlining |
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Sorry for the delay...
On Thu, 29 Dec 2005, Ingo Molnar wrote:
> > * Nicolas Pitre <nico@cam.org> 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.
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