Re: First set of OSDL Shared Mem scalability results, some wierdness ...

Tom Lane wrote:
> Kevin Brown <kevin@sysexperts.com> writes:
> > Hmm...something just occurred to me about this.
>
> > Would a hybrid approach be possible?  That is, use mmap() to handle
> > reads, and use write() to handle writes?
>
> Nope.  Have you read the specs regarding mmap-vs-stdio synchronization?
> Basically it says that there are no guarantees whatsoever if you try
> this.  The SUS text is a bit weaselly ("the application must ensure
> correct synchronization") but the HPUX mmap man page, among others,
> lays it on the line:
>
>      It is also unspecified whether write references to a memory region
>      mapped with MAP_SHARED are visible to processes reading the file and
>      whether writes to a file are visible to processes that have mapped the
>      modified portion of that file, except for the effect of msync().
>
> It might work on particular OSes but I think depending on such behavior
> would be folly...

Yeah, and at this point it can't be considered portable in any real
way because of this.  Thanks for the perspective.  I should have
expected the general specification to be quite broken in this regard,
not to mention certain implementations.  :-)

Good thing there's a lot of lower-hanging fruit than this...



--
Kevin Brown                          kevin@sysexperts.com