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тАО09-09-2008 05:45 AM
тАО09-09-2008 05:45 AM
Before I put global buffers on a particular indexed file, the MONITOR RMS /FILE= /ITEM=CACH shows "Loc Buf Cache Attempt Rate" (with hit percent around 99%) and shows zero "Glo Buf Cache Attempt Rate". After I added global buffers to the file, I now see global buffer attempt rate (close to 99% hit rate). However, I still see almost the same amount of Local Buffer cache attempt rate (with hit rate of 0%). So now the total attempt rate (local + global) seems double what it was before global buffers was added. What seems suspicious to me is that the local and global attempt rates are numerically very close to each other. I suspect this is due to the nature of the report program running against the file, but I haven't looked at the code yet. I was wondering if someone had a ready explanation for this type of behavior.
(attached is monitor rms output)
OVMS Alpha 8.3 with fairly recent patches.
Solved! Go to Solution.
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тАО09-09-2008 06:08 AM
тАО09-09-2008 06:08 AM
Re: Increased overhead on RMS Global Buffers?
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тАО09-09-2008 06:36 AM
тАО09-09-2008 06:36 AM
Solutionthe following explanation may answer your question:
If only Global buffers are set, the Local Cache Hit Percent will be zero because VMS looks in the Local Buffers before looking in the Global buffers. If the requested data is not in the Local Buffers, the Global buffers are searched for the data. If the data is not in the Global buffers, VMS gets the data from disk. VMS then puts the data in a Global buffer since Global buffers were the last place VMS checked for the data.
Found at:
http://www.ttinet.com/tti/SECRETS_FILE_IO.HTML
Volker.
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тАО09-09-2008 06:43 AM
тАО09-09-2008 06:43 AM
Re: Increased overhead on RMS Global Buffers?
from looking at your first set of MONITOR RMS data, I'd say you've saved 50% of physical IOs. You get about the same 'attempt rate' with half the amount of read-I/Os.
Volker.
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тАО09-09-2008 06:52 AM
тАО09-09-2008 06:52 AM
Re: Increased overhead on RMS Global Buffers?
"Even if global buffers are used, a minimal number of local buffers should be
requested, because, under certain circumstances, RMS may need to use local
buffers. When attempting to access a record, RMS looks first in the global buffer
cache for the record before looking in the local buffers; if the record is still not
found, an I/O operation occurs."
I will look into using local buffers, too. Thanks.
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тАО09-09-2008 07:30 AM
тАО09-09-2008 07:30 AM
Re: Increased overhead on RMS Global Buffers?
There have been a lot of modifications to Global Buffers code since then, and Edgar did say he was using 8.3 and fairly recent patches.
Hein will probably respond, and he should know.
At any rate, if the file is being shared for update, you are almost certainly better off with global buffers.
Have you done some tests of elapsed time when there are multiple writers to the same file?
Jon
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тАО09-09-2008 07:43 AM
тАО09-09-2008 07:43 AM
Re: Increased overhead on RMS Global Buffers?
Jon wrote
>>>
At any rate, if the file is being shared for update, you are almost certainly better off with global buffers.
<<<
to specify "almost certainly":
On 8.3 (with XFC enabled) GLOBAL buffers start being advantaguous AS SOON AS the file is accessed SIMULTANUOUS with at least one of those accesses with UPDATE intent.
(and if you think about it, that is exactly what logic would predict)
hth
Proost.
Have one on me.
jpe
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тАО09-09-2008 02:10 PM
тАО09-09-2008 02:10 PM
Re: Increased overhead on RMS Global Buffers?
O yeah of little faith! :-)
Just start use them and make your bosses happy.
Be sure to have before and after performance picture (T4... notably for LOCK and KERNEL MODE activity, and my HOT_FILES tool that you could have picked up from BootCamp or HP Tech Forum proceedings),.
>> RMS /FILE= /ITEM=CACH shows "Loc Buf Cache Attempt Rate" (with hit percent around 99%)
So you can NOT expect global buffers to significantly reduce IO rates.
It is hard to improve on 100%. :-)
Stil, going from 98% to 99% can mean 1/2 the IOs.
But most importantly, you can expect to have obtained a significant lock traffic reduction which is a scares (sp?), serializing, resource.
>> I still see almost the same amount of Local Buffer cache attempt rate
Right. It TRIES the local buffer first, then the global, as other explained.
It will use the first found, but load to the global buffer UNLESS the target is accessed for write-intend.
I explain this in my RMS sessions, which are floating on the web.
>> due to the nature of the report program
It almost sounds (99%) that the program is re-reading a lot of data, in which case the better helper is to teach it to remember the records. But it could also be reading record after record, many from the same bucket/buffer doing only 1 IO every 100+ records for a fresh bucket full of records.
Those are an impressive rates though as per attachment. That should make a dent in CPU usage. Try and use my RMS_STATS program (OpenVMS freeware V6, or more recent version from me). It will show more counters, and in a single picture.
To reduce a tiny bit of CPU and memory access overhead, you may want to reduce the number of local buffer when global buffers are present (FAB$W_GBC), but that typically need program changes.
You failed to indicate HOW MANY global buffer were applied. With hundreds of IOs/sec you may want to go aggresive. Think THOUSANDS, not hundreds.
Mind you... when RMS counts IO, those are IO requests which are not unlikely to have been resolved by the XFC. So the actual IO rate to the disk might not have changed.
I pretty much only expect LOCK and KERNEL mode reduction, which out to be significant at 65K/second. So as I opened, check to for T4, hotfile or rms_stats detail infor to see the real gain.
The end user impact may have been surprisingly low, ut you may have help the system as a whole a lot.
Hope this helps,
Hein van den Heuvel ( at gmail dot com )
HvdH Performance Consulting
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тАО09-09-2008 02:37 PM
тАО09-09-2008 02:37 PM
Re: Increased overhead on RMS Global Buffers?
Even with global buffers, a buffer to be modified is copied to the process' local buffer and when requested again passes through the disk back into the global buffer. That why writes don't make a difference between local and global buffers.
Ah, what was the question?
/Guenther
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тАО09-09-2008 03:19 PM
тАО09-09-2008 03:19 PM
Re: Increased overhead on RMS Global Buffers?
/Guenther