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AIX (DSCR) and oracle prefetching

Posted on 2014-01-22
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what is AIX (DSCR) prefetching and how it works with Oracle?
our prod DB is showing:
dscrctl -q
Current DSCR settings:
        Data Streams Version  = V2.06
        number_of_streams     = 12
        platform_default_pd   = 0x4 (DPFD_MEDIUM)
       os_default_pd         = 0x0 (DPFD_DEFAULT)

what does this mean?
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Question by:it-rex
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woolmilkporc earned 2000 total points
ID: 39801187
Hi,

DSCR is hardware memory prefetch. It sometimes can improve the performance of applications, if they reference memory sequentially.

Oracle does not often perform sequential memory access, in fact Oracle's memory access is said to be "irregular", so aggressive prefetching can have an adverse effect on performance because the prefetched lines might not be referenced. Your setting is not very aggressive, though.

I must admit that I'm not a true Oracle tuning specialist, so be careful with the following.

If you're currently experiencing bad performance you could run this command

dscrctl -b -n -s 1

which will turn off memory prefetching (-b = persist across reboots, -n = apply changes immediately, -s 1 = set the dscr value to 1, which is DFPD_NONE).
Please watch the DB very thoroughly for the effect of this setting, and should you see a decrease in performance turn prefetching on again with

dscrctl -b -n -s 0

which will go back to your defaults.

wmp
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by:it-rex
ID: 39801230
so what is the difference between
 os_default_pd         = 0x0 (DPFD_DEFAULT)
and the default of
 os_default_pd         = 0x5 (DPFD_DEFAULT)

?
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by:it-rex
ID: 39801242
Sir..
Also i'm posting few AIX specific Q's and I really like to see your input about them..
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LVL 68

Expert Comment

by:woolmilkporc
ID: 39801328
0x0 sets os_default to platform_default, which is 0x4 DPFD_MEDIUM fot the machine shown in your question.

0x5 is not DPFD_DEFAULT, and also not platform_default, at least not on the machine you showed in your Q. There are machines whose platform_default is 0x5, but DFPD_DEFAULT is always 0x0!
0x5 is DFPD_DEEP, which is more aggressive than DPFD_MEDIUM
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by:it-rex
ID: 39801351
so what do you suggest here?
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by:woolmilkporc
ID: 39801371
Leave it at 0x0 (the default which is DFPD_MEDIUM in your case) or turn it off according to my first comment - please read it again - in full!
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