Re: Are Linux pipes slower than the FreeBSD ones ?

[Eric Dumazet]

Nick Piggin a écrit :
> On Wednesday 05 March 2008 20:47, Eric Dumazet wrote:
>   
> > David Miller a écrit :
> >     
> > > From: Antipov Dmitry <dmantipov@xxxxxxxxx>
> > > Date: Wed, 05 Mar 2008 10:46:57 +0300
> > >
> > >       
> > > > Despite of this obvious fact, recently I've tried to compare pipe
> > > > performance on Linux and FreeBSD systems. Unfortunately, Linux
> > > > results are poor - ~2x slower than FreeBSD. The detailed description
> > > > of the test case, preparation, environment and results are located
> > > > at http://213.148.29.37/PipeBench, and everyone are pleased to look
> > > > at, reproduce, criticize, etc.
> > > >         
> > > FreeBSD does page flipping into the pipe receiver, so rerun your test
> > > case but have either the sender or the receiver make changes to
> > > their memory buffer in between the read/write calls.
> > >
> > > FreeBSD's scheme is only good for benchmarks, rather then real life.
> > >       
> > page flipping might explain differences for big transferts, but note the
> > difference with small buffers (64, 128, 256, 512 bytes)
> >
> > I tried the 'pipe' prog on a fresh linux-2.6.24.2, on a dual Xeon 5120
> > machine, and we can notice that four cpus are used (but only two threads
> > are running on this benchmark)
> >     
>
> One thing to try is pinning both processes on the same CPU. This
> may be what the FreeBSD scheduler is preferring to do, and it ends
> up being really a tradeoff that helps some workloads and hurts
> others. With a very unscientific test with an old kernel, the
> pipe.c test gets anywhere from about 1.5 to 3 times faster when
> running it as taskset 1 ./pipe
>
>
>   
> > # opreport -l /boot/vmlinux-2.6.24.2 |head -n 30
> > CPU: Core 2, speed 1866.8 MHz (estimated)
> > Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a
> > unit mask of 0x00 (Unhalted core cycles) count 100000
> > samples  %        symbol name
> > 52137     9.3521  kunmap_atomic
> >     
>
> I wonder if FreeBSD doesn't allocate their pipe buffers from kernel
> addressable memory. We could do this to eliminate the cost completely
> on highmem systems (whether it is a good idea I don't know, normally
> you'd actually do a bit of work between reading or writing from a
> pipe...)
>
>
>   
> > 50983     9.1451  mwait_idle_with_hints
> > 50448     9.0492  system_call
> > 49727     8.9198  task_rq_lock
> > 24531     4.4003  pipe_read
> > 19820     3.5552  pipe_write
> > 16176     2.9016  dnotify_parent
> >     
>
> Just say no to dnotify.
>
>
>   
> > 15455     2.7723  file_update_time
> >     
>
> Dumb question: anyone know why pipe.c calls this?
>
>   
Because pipe writer calls write() syscall -> file_update_time() in kernel
while pipe reader calls read() syscall -> touch_atime() in kernel

inode i_mtime, i_ctime, i_atime and i_mutex fields share same cache 
line, so nothing we can improve to avoid cache line ping pong.





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