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CPU Capacity Planning

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Ian Dennison_1
Honored Contributor

CPU Capacity Planning

My situation: We are rehosting from N-Classes to newer Servers that have faster spec CPUs so (of course) the decision was made to purchase less of them.

My dilemma: The calculations for CPU capacity were done on TPM figures

eg 4 x old CPUs (4000TPM) = 16000
2 x new CPUs (8000TPM) = 16000

All other aspects of the systems being run (ie number and type of applications, user load) will remain the same.

Question: Can this calculation be validly done in a straight line? What I remember from Perf&Tuning is that reducing the CPUs will increase the amount of System Mode time the System has to spend in swapping registers, handling page faults, etc. There must be other aspects of architecture as well that will be affected by reducing CPU count, any pointers?

Look forward to your input, thanks, Ian
Building a dumber user
13 REPLIES
Mark Grant
Honored Contributor

Re: CPU Capacity Planning

I don't think this works too well. You need to consider your normal load. If, for example, you have one job that hogs CPU for eight hours you pretty much have only one CPU to play with. My preference would be for more CPUS
Never preceed any demonstration with anything more predictive than "watch this"
Ian Dennison_1
Honored Contributor

Re: CPU Capacity Planning

Good point, Mark.

PRI_QUEUE will increase as more processes vie for less CPUs. Long-runners are an issue as the applications we use are not truly multi-threaded.

Keep it coming, thanks chaps! Ian
Building a dumber user
Jean-Luc Oudart
Honored Contributor

Re: CPU Capacity Planning

Ian,

as mentioned this is not as straigth forward.
TPM is just an indication, and you would have to have "at least" the same TPM. If you run an upgrade, you would increase this number (keep some capacity).

The real answer would be benchmark. Unfortunately, we rarely have this chance when we neeed to upgrade the hardware.

For instance, if you have hundreds of user processes, more cpu might be better, but if you run batch processing, faster cpus works better. (It did for me).
So, you have to balance user response time and batch window.
Again this depends on the type of application you are running.

Regards,
Jean-Luc
fiat lux
Pete Randall
Outstanding Contributor

Re: CPU Capacity Planning

Ian,

I have no real advice for you (yet) - only sympathy. My management decided that, because we had 6x440MHz CPUs in our N-class, we could get away with 4x875MHz chips in the new rp7410, 1 of which is claimed by a second VPAR, leaving 3! The math works, I guess: 3 CPUs that are twice as fast as the 6 CPUs they're replacing, but I'm not sure how the actual performance is going to be. I'll let you know in a month or so.

I do have to say, though, that with the prevalence of per CPU licensing, there will be a lot of pressure to keep CPU counts down simply for economy's sake. It's going to put more pressure on us to tune properly because the days of overpowering an application with horsepower are going to be disappearing.


Pete

Pete
Steve Lewis
Honored Contributor

Re: CPU Capacity Planning


1. See the answers to my question here:
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=542462

2. The only gotcha that I found was with moving to a multi-cell rp8400 in one npar. Where a running process is migrated from a cpu on one cell to a cpu on another cell, there is an extra overhead of data migration down the backplane, with associated latches/locks, cache clearing, memory moves etc. Its fairly marginal, but it does exist. I lost the thread to do with this, lets try a search...





Nicolas Dumeige
Esteemed Contributor

Re: CPU Capacity Planning

Hello,

The benefit of faster CPU against more CPU also depend on the coding of the application the box is hosting.

I used to work on Oracle Express wich is mono-threded when it build the database but multi-threded for user request.

Mono-threaded apps benefits from fastest CPU.
Multi-threaded apps will sometime perform better in a multi CPU environnement.

Even for multi-threaded apps, the benefit is no garantee : is you see a lot of mutex for instance, you have a sitution where all thread are fighting for resource and the overhead of this management is not to be ignore.

To me, the knowledge of the sofware you run is the key. If you have several apps with different needs... well... good luck !

Cheers

Nicolas
All different, all Unix
Mott Given
Frequent Advisor
Solution

Re: CPU Capacity Planning

I'd use the SPEC CPU ratings at http://www.spec.org/cpu2000/results/cint2000.html to get the relative CPU speed ratings between your current CPU and the one you are thinking of upgrading to. The SPEC rating for a multiprocessor will be less than the uniprocessor rating multiplied by the number of processors in the multiprocessor - if you look at various ratings in the table you can get a good idea what the ratio should be.

To get the relative throughput ratings between 2 CPU models, go the following link.
http://www.spec.org/cpu2000/results/rint2000.html

Mott Given
Jeff Schussele
Honored Contributor

Re: CPU Capacity Planning

Hi Ian,

Off topic - I know - but I'm curious to know if you're still in the service to the Queen? I'm sure, across the pond, that it's as risky there to be a serviceman as it is here.
It distresses me greatly that several friends of mine have been informed by our govt that their kid's deployment in Iraq has been extended by 90 more days after slogging out a full year in that hell-hole.
Am I the only one that sees the irony & hypocrisy of a President that demands more from their Guardsmen than they agreed to than they couldn't even live up to themself?

Rgds,
Jeff (OK - off soapbox now)
PERSEVERANCE -- Remember, whatever does not kill you only makes you stronger!
Ian Dennison_1
Honored Contributor

Re: CPU Capacity Planning

Jeff,

[Off topic] Yes, I am still in the service of the Queen over here. Have been busy at work lately, so have been keeping a low profile.

I would love to put forth my views on the current situation in Iraq; however I am reminded of the pearls of wisdom
from my Sgt-Major on Recruit Course - "you are a soldier, therefore you are specifically paid NOT! to have an opinion!".
Remember, its not paranoia when they really are watching you! :^)

Am holding thumbs that everyone deployed to Iraq and Afghanistan makes it back home safe and well.

Cheers, Ian
Building a dumber user
Paula J Frazer-Campbell
Honored Contributor

Re: CPU Capacity Planning

Ian

Please also be aware that you can easily be in breach of Queens Regs (section 69 and 70) if you voice an opinion that may reflect against Queen and Country.

Paula

Ex WO1 (ASM) REME


If you can spell SysAdmin then you is one - anon
Jeff Schussele
Honored Contributor

Re: CPU Capacity Planning

Again, I'm sorry I took it off-topic.
But I would first & foremost like to thank you & praise you for your service.
I'm just frustrated as a citizen of a free country that I'm priviledged to be a citizen of - God bless the USA. I just think we should be putting *every* dollar & effort into getting 'bin Laden & it just pains me that that your * I's govt could be so easily distracted by a such a gnat like Hussein.
Of course that's just my opinion...

Bset Rgds,
Jeff

P.S. Sounds like it's time for Princess to re-enlist. Lord knows they need *somebody* with some common sense. We definitely need somebody that knows how to kill a man several dozen ways.... 8~()
PERSEVERANCE -- Remember, whatever does not kill you only makes you stronger!
Paula J Frazer-Campbell
Honored Contributor

Re: CPU Capacity Planning

Jeff

ROTFLMFAO

XXX

Paula
If you can spell SysAdmin then you is one - anon
Jeff Schussele
Honored Contributor

Re: CPU Capacity Planning

Luv ya 2 sweetheart.

JS
PERSEVERANCE -- Remember, whatever does not kill you only makes you stronger!