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List:       hpux-devtools
Subject:    RE: HPUX-DEVTOOLS: disadvantage of EXEC_MAGIC executable ?
From:       Mike Stroyan <stroyan () hpstryn ! fc ! hp ! com>
Date:       2001-04-27 17:58:32
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| I checked the white paper and found the following;
| --- on page 18
| EXEC_MAGIC text cannot be shared between multiple processes. To 
| overcome this limitation, the EXEC_MAGIC implementation allows 
| read-only aliasing; multiple addresses can map the same physical
|  page.
| --- on page 19
| Impact and Performance
|  More memory and swap space is available when running multiple
|  copies of the same EXEC_MAGIC executable because unmodified 
| text pages are now shared.
| ---
| 
| According to above, it seems your point has been improved.  
| Could you please give me the explanation about it ?

  Yes, multiple processes can share the same RAM for pages in the text area
of an EXEC_MAGIC executable.  They only need a separate page in RAM if they
write to the text area.  That is something that a SHARE_MAGIC executable
can never do.  Multiple processes will still have separate page entries in
the instruction TLB.  That is one more pressure on the ITLB, in combination
with the limit to 4K pages for EXEC_MAGIC text areas.  That could reduce
performance if the working set was larger than the ITLB size.  It is very
unlikely to cause actual trouble.  Most processes are light users of the
instruction TLB.  It is really unlikely for code execution to hop around
between very many pages.

Mike Stroyan, mike_stroyan@fc.hp.com
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