Re: MMAP Buffers

=?utf-8?q?Rados=C5=82aw_Smogura?= <rsmogura(at)softperience(dot)eu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> Sunday 17 April 2011 01:35:45
>> ... Huh? Are you saying that you ask the kernel to map each individual
>> shared buffer separately? I can't believe that's going to scale to
>> realistic applications.

> No, I do
> mrempa(mmap_buff_A, MAP_FIXED, temp);
> mremap(shared_buff_Y, MAP_FIXED, mmap_buff_A),
> mrempa(tmp, MAP_FIXED, mmap_buff_A).

There's no mremap() in the Single Unix Spec, nor on my ancient HPUX box,
nor on my quite-up-to-date OS X box. The Linux man page for it says
"This call is Linux-specific, and should not be used in programs
intended to be portable." So if the patch is dependent on that call,
it's dead on arrival from a portability standpoint.

But in any case, you didn't explain how use of mremap() avoids the
problem of the kernel having to maintain a separate page-mapping-table
entry for each individual buffer. (Per process, yet.) If that's what's
happening, it's going to be a significant performance penalty as well as
(I suspect) a serious constraint on how many buffers can be managed.

regards, tom lane