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IDS Forum

Re: What means "nonblocked checkpoint" ?!

Posted By: Art Kagel
Date: Tuesday, 12 August 2008, at 10:28 p.m.

In Response To: What means "nonblocked checkpoint" ?! (MARS CHEN)


More likely this is a problem I have tried to point out to the engine
architects since 7.24 days.

The problem may not be that your transaction was blocked, technically it was
not. Hhow many CPU VPs do you have configured? Probably just one. While
the engine is gathering the list of pages that need to be flushed to disk
for the checkpoint, the checkpoint thread runs in CPU VP #1 without
relinquishing its hold on the VP until the list is complete. This is the 3
seconds at the beginning of the checkpoint. It's not so much that you were
blocked, but that you could not run while the checkpoint thread was hogging
the main (and for you perhaps the only) CPU VP.

In earlier versions of IDS (and to my knowledge this was never fixed - the
engineers just claimed that Fuzzy Checkpoints made it a non-issue so there
was nothing to fix) even if you had many CPU VPs, all of the active threads
running in CPU VP #1 were suspended during this first checkpoint phase and
were not released to the ready queue where they might be picked up by other
CPU VPs which are not blocked. So, any threads running in CPU VP #1 would
block during the beginning of any checkpoint - even if they were not making
updates and not in a critical section. If you are running only a single SHM
poll thread in CPU VP #1 that means that this VP is likely running most of
your active threads!

I delivered test code VERY similar to yours that demonstrated the problem
about 10 years ago. You might try the same test without the updates and see
if the session is still pausing as it did when I last tested this so long

To the engine developers: I propose the same two solutions I proposed when I
first raised this one, either:

- Release all threads running in CPU VP #1 before starting the checkpoint

so that they can run in other CPU VPs if any, or better:

- Run the checkpoint thread in the ADM or MSC VP so that they do not

block or shut out anything!


On Tue, Aug 12, 2008 at 9:52 PM, MARS CHEN <> wrote:

> Dear All...
> I am now testing IDS 11.5 in RHEL 5, since I am interesting in
> nonblocking checkpoint , I config PHYSFILE 900M , BUFFERS 600M ,
> and run a esql/c ap , which do endless loop update a table ,
> 50,000 rows of them are updated each time , size 300 bytes each row !!!!
> Because both physfile and buffers are large enough , the onstat -F
> showes no chunk writes and the performance looks fine , and then
> I type "onmode -c" to do the checkpoint , the esql/c ap take a break
> for about 3 seconds(I printf the loop counter ... it delayed for 3 secs
> while
> checkpoint happens),the online.log showes "duration was 4 seconds."
> and "Checkpoint Statistics - Avg. Txn Block Time 0.000 ,#Txns blocked 0..",
> onstat -g ckp showes also "Block Time 0" !!!
> It is not what I think about "nonblocking checkpoint" , seems look like
> it still block the transaction while do checkpoint , It is my
> misunderstanding for this concept ?!
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.

Art S. Kagel
Oninit (
IIUG Board of Directors (

Disclaimer: Please keep in mind that my own opinions are my own opinions and
do not reflect on my employer, Oninit, the IIUG, nor any other organization
with which I am associated either explicitly or implicitly. Neither do those
opinions reflect those of other individuals affiliated with any entity with
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