shared memory segment size alignment

shared memory segment size alignment

[umut aymakoglu]

Hi -
In some kernel versions the SHMLBA is defined as
1MB(1024*1024) and in some as 16K. What would be the
difference between aligning the shared memory segment
size on 1MB versus 16K before calling shmget()? Would
there be any performance gain/lose? We are aligning
the segment address on 1MB.

thanks,
Umut

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Re: shared memory segment size alignment

[David Mosberger]

>>>>> On Fri, 16 Jan 2004 10:31:11 -0800 (PST), umut aymakoglu <umutaymak@yahoo.com> said:

  umut> Hi - In some kernel versions the SHMLBA is defined as
  umut> 1MB(1024*1024) and in some as 16K. What would be the
  umut> difference between aligning the shared memory segment size on
  umut> 1MB versus 16K before calling shmget()? Would there be any
  umut> performance gain/lose? We are aligning the segment address on
  umut> 1MB.

The ia64 architecture allows CPUs to have a performance penalty when
there is virtual aliasing below a 1MB boundary.  So if you don't align
to 1MB, it _could_ run slower.  No existing processor models do that,
but future models might.

	--david