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From (Jens-Uwe Mager)
Subject Re: Question: mmap und shared mem
Date Fri, 21 Aug 1998 15:24:24 GMT

>Good guidelines. And what about the maximum size of a shared memory segment of
>about 131072 bytes as mentioned in some books (for instance in Stevens)?  They
>say that the size (as used with shmget) should be below 131072 bytes and that
>each process can only shmget up to 6 segments. But in the manpages of the
>current Unix derivates I do not see such a limit. Do you have any experiences
>here, too? 

The maximum sizes of shared memory for shmat are indeed limited, but
can be tuned in most instances. All database vendors (Informix,
Oracle...) do rely on large shared memory segments for their database
servers. Any commercial Unix vendor will provide some tunables here to
be able to use these databases. The number of segments can be tuned as
well, but I believe the number of segments possible is in some
circumstances very limited. For example AIX before version 4.3 did
implement shmat by loading a segment register in the PowerPC CPU. The
CPU does only have 16 segment registers, and several are reserved for
the OS, so one had only 10 shmat segments available. I would believe
it is better to tune the system to allow for larger segments as to
attach multiple segments to get around the size restriction.

Jens-Uwe Mager

HELIOS Software GmbH
Steinriede 3
30827 Garbsen

Phone:		+49 5131 709320
FAX:		+49 5131 709325

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