Re: Fw: [vorbis-dev] ogg123: shared memory by mmap()

> > > The patch below adds:
> > >
> > > - acinclude.m4:  A new macro A_FUNC_SMMAP to check that sharing
> > pages
> > >   through mmap() works.  This is taken from Joerg Schilling's star.
> > > - configure.in:  A_FUNC_SMMAP
> > > - ogg123/buffer.c:  If we have a working mmap(), use it to create
> > >   a region of shared memory instead of using System V IPC.
> > >
> > > Works on BSD.  Should also work on SVR4 and offspring (Solaris),
> > > and Linux.
> 
> This is a really bad idea performance wise.  Solaris has a special
> code path for SYSV shared memory that doesn't require tons of swap
> tracking structures per-page/per-process.  FreeBSD also has this
> optimization (it's off by default, but should work since FreeBSD
> 4.2 via the sysctl kern.ipc.shm_use_phys=1)

> 
> Both OS's use a trick of making the pages non-pageable, this allows
> signifigant savings in kernel space required for each attached
> process, as well as the use of large pages which reduce the amount
> of TLB faults your processes will incurr.

That is interesting.  BSDi has SysV shared memory as non-pagable, and I
always thought of that as a bug.  Seems you are saying that having it
pagable has a significant performance penalty.  Interesting.

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