Horton

Re: MMAP Buffers

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Radosław Smogura <rsmogura@softperience.eu>
Cc: Peter Eisentraut <peter_e@gmx.net>, Greg Stark <gsstark@mit.edu>, Robert Haas <robertmhaas@gmail.com>, Greg Smith <greg@2ndquadrant.com>, Joshua Berkus <josh@agliodbs.com>, Andrew Dunstan <andrew@dunslane.net>, Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>, PG Hackers <pgsql-hackers@postgresql.org>
Date: 2011-04-17T15:48:56Z
Lists: pgsql-hackers 
=?utf-8?q?Rados=C5=82aw_Smogura?= <rsmogura@softperience.eu> writes:
> Tom Lane <tgl@sss.pgh.pa.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