Replay now available: A New View of the Multi-User Start-Up Options

[Lenn Thompson]

This video is the replay from our December 2022 Datacom CADRE Community webcast "A New View of the Multi-User Start-Up Options," hosted by @Kevin Shuma.

I've also attached Kevin's presentation to this post. 

If you have any questions, please drop them below in the comments. 

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Lenn Thompson
Community Manager, Mainframe Division
Broadcom Inc.
[e-mail]
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Attachment(s)

Datacom Multi User Start Up Options Community Webcast 120...pptx   19.85 MB 1 version

[Owen Williams]

Hi Kevin,

Great session. This certainly is a new and valuable perspective.

I have one question...
You are recommending sites to code FLEXPOOL 10,10,10 in the MUF start-up options as a "default" set.
Many years ago, I received a recommendation (I think from Bill Stepler) to not specify FLEXPOOL as a MUF start-up option because...

1. Maintaining the separate FLEXPOOL buffer pool was (small) additional overhead to the buffer management code.
2. FLEXPOOL (via console command or via start-up option) is useful for experimentation, but once the optimum size is determined, all FLEXPOOL buffers should be "merged" into the static SYSPOOL/DATAPOOL buffer pools.
3. Even when FLEXPOOL is not specified as a MUF start-up option, the Console FLEXPOOL command is still available, to add further buffers to cope with emergencies or spikes.  Therefore there is no need to "reserve" 10 buffers in each pool.

If the above is still correct, then it would seem that coding FLEXPOOL 10,10,10 adds the buffer management overhead, with no benefit for the 99% of the time when a meaningful FLEXPOOL value would not subsequently be applied.

These days (with much larger SYSPOOL/DATAPOOL and the additional shared buffer pools + Buffer Pool by Area) we don't specify FLEXPOOL as a MUF start-up option, and rarely use it as a console command.

Have I misunderstood (or maybe the code has changed since that previous advice)?

Cheers,
Owen

[Kevin Shuma]

Owen,

I hope you are having a great holiday season!

I reviewed your points and wanted to provide a response to the group.
As usual I really appreciate the time you take to review all the sessions
and materials. Your input is greatly valued.

After getting your email, I set-up a specific benchmark to test out your
points.
After a few hours of testing, I would have to agree with your input.

Specifically:

#1. *Maintaining a FLEXPOOL does take a slight bit of overhead.*
The primary buffer pools are allocated together as a fixed set of memory.
The FLEXPOOL buffers are allocated later in the startup cycle in a memory
area that can be expanded.
For MUF to maintain both pools a slight bit of overhead is needed.
In my tests, I could only see a measurable difference when I used large
FLEXPOOL amounts (10,000). With smaller FLEXPOOL amounts (10) it was not
measurable but it would still be there.
Notes: 1. In all cases, the IOs required to do the work were the same when
the total number of buffers allocated was the same (in either standard
pools or standard plus FLEXPOOL).
2. The good part was that the GP CPU remained very consistent
in all tests, the only thing that varied was some additional zIIP CPU with
large FLEXPOOL numbers.
3. There are a lot of variables that play into benchmarking
such as total buffers, COVERed, etc. So it's not an exact science.
I did make multiple runs of a very consistent workload
using multiple combinations of MUF options.
I believe if I had used a more varied workload it would
have been even harder to see a difference.

#2. *FLEXPOOL should be used for experimentation* and once you have the
right number, you should merge into standard pools.
I also give this advice to most sites..
FLEXPOOL does allow sites with a limited recycle (PRODUCTION schedule) to
increase buffers when needed without taking an outage.


#3. *FLEXPOOL can be added via the console even if it is not in the
MUF startup options*.
That is correct, I reread the MUF Startup options text and it is a bit
misleading. I will get an update made to the doc.
The only thing you can not do is add FLEXPOOL for a buffer pool that is not
in the standard pools (SYSPOOL, DATAPOOL).
Since MUF requires values for IXX, DXX, DATA buffers the only pool this
really affects is DATA2 pool.


When we provide this session in the future, we will update the text
appropriately.

Thanks for the input,

Kevin
Kevin Shuma
Director of Engineering
*Broadcom Software*
5465 Legacy Drive, Plano, TX 75024-3106
Mobile: [phone]
[e-mail]