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Adjusting lru_min_dirty and lru_max_dirty when an instance is online

  • 1.  Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Thu March 03, 2022 11:41 AM
    Hi, is there any way to effect a change to LRU min/max dirty while an instance is online?

    I am aware of AUTO_LRU_TUNING which does adjust these parameters. The behaviour of this when turned on is to set all pools to 50%/60% then adjust them as per the rules in the manual. Switching it off again seems to lock the values.

    Thanks in advance,
    Ben.

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    Benjamin Thompson
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  • 2.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Thu March 03, 2022 06:51 PM
    I always modify parameters and restart the database

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    ZhiWei Cui
    GBASE
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  • 3.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Fri March 04, 2022 08:56 AM

    Hi Ben et al,

    Please vote up this request:

    https://ibm-data-and-ai.ideas.ibm.com/ideas/INFX-I-455

    Allow Bufferpool Min_lru_dirty and Max_lru_dirty Threshold values to be altered dynamically

    Regards,
    David.



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    David Williams
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  • 4.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Mon March 07, 2022 04:57 AM
    Hi David,

    I think that RFE answers my question!

    @ZhiWei Cui restarting the database is not an option available to me at present.

    As LRU tuning usually requires a few attempts to get right it would be useful to be able to dynamically tune it and clearly the facility is there because auto-tune uses it.

    Thanks, Ben.​

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    Benjamin Thompson
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  • 5.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Mon March 07, 2022 10:41 AM
    I just posted 2 comments on such a feature besides upvoting it.
    • Expressed surprise that we haven't had it for years already.  Unfortunately, my search for a paycheck has led me astray from the Informix & DBA world so I've been a trifle out of touch.
    • Add a feature request on top of that:

      On the other hand, I would complicate matters by having a different parameter, say:

      AUTOTUNELRU 1/0 # To enable or disable auto LRU tuning

      and having yet another onmode option to dynamically enable/disable it at the discretion of the DBA.



    ------------------------------
    Jacob Salomon
    ---
    Nobody goes there anymore, it's too crowded.  --Attr: Yogi Berra
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  • 6.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    InnerCircle
    Posted Mon March 07, 2022 02:35 PM

    Our goal over time has been to replace manual knobs with autonomics wherever it made sense to us, so it's a little painful to go in the opposite direction, but we'll consider bumping up IFMX-I-455. Meanwhile I'm going to trust you guys with an undocumented, unsupported feature that should allow you to do exactly what you're asking for. I have not personally tested this method in a benchmark and cannot vouch for it in a production environment, but I see no reason to warn you off of it for test systems because it makes use of the same internal routines that automatic LRU tuning uses. If you try this and let me know whether it works for you, hacky as it is, that will inform decisions we eventually make on IFMX-I-455.

    This method assumes you're beginning with AUTO_LRU_TUNING off. Say you want to set lru_min to 10 and lru_max to 20 for all buffer pools:

    onmode -wm AUTO_LRU_TUNING="1,min=10,max=20"
    onmode -wm AUTO_LRU_TUNING=0

    Note that even though we finish with AUTO_LRU_TUNING off again, the new min and max settings should remain. Check with onstat -R.

    Say you want to customize lru_min and lru_max for a specific buffer pool. Again these commands will only work if AUTO_LRU_TUNING is off to begin with:

    onmode -wm AUTO_LRU_TUNING="1,bpool=0,min=60,max=70"
    onmode -wm AUTO_LRU_TUNING=0

    Note that buffer pools are numbered 0-7 on 2k systems and 0-3 on 4k systems. Pool numbers are fixed for the different page sizes. I.e. on a 2k system even if you have only two buffer pools: a 2k pool and a 16k pool, their numbers are 0 and 7, respectively.

    I hope you find the automatic tuning algorithm works as well as manual tuning in terms of I/O, checkpoint, and overall performance. If manual tuning is significantly better in some scenario though, I'd be interested to hear about it. Have fun and send feedback to me or your customer advocate if you have one.

    -jc



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    John Lengyel
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  • 7.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    IBM Champion
    Posted Mon March 07, 2022 05:07 PM
    JC:

    Using onmode -wm AUTO_LRU_TUNING to adjust the lru_min/max_dirty settings seems to have worked fine on my server. Very cool thanks. Obviously ultimately we need to be able to adjust them per buffer cache. This workaround changes all caches.

    Art

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    Art S. Kagel, President and Principal Consultant
    ASK Database Management Corp.
    www.askdbmgt.com
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  • 8.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Mon March 07, 2022 05:47 PM

    Add bpool=0-7 or 0-4 ( divided bufferpool size by page size)

     






  • 9.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Mon March 07, 2022 05:59 PM

    onmode -wm AUTO_LRU_TUNING="1,bpool=0,min=5,max=10"

     

    informix@aragorn[live]: onstat -R | egrep "(^start|^  .*[1-9])"

    start clean at  10.000% (of pair total) dirty, or 781 buffs dirty, stop at

       5.000%

    start clean at  60.000% (of pair total) dirty, or 586 buffs dirty, stop at

      50.000%

    start clean at  60.000% (of pair total) dirty, or 281 buffs dirty, stop at

      50.000%

     






  • 10.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    Posted Tue March 08, 2022 11:17 AM
    Hi John,

    Thanks for the reply. I need some time to evaluate this and the effects of AUTO_LRU_TUNING properly but certainly automatic LRU tuning is something I am interested in. Right now LRU tuning is an iterative exercise, something Informix itself could accomplish much better.

    So for us to adopt automatic tuning it would come down to how good the implementation is and what testing we have done (currently very little to none).

    I need to ensure adequate application response times for our applications through checkpoints. Even though checkpoints are non-blocking and we have a fast all-flash storage array, they can be quite noticeable at the application layer, at least when we draw a graph of response times for certain queries. Very occasionally the response from Informix exceeds application timeouts. It is not possible to measure the response time at the server so my concern would be how you could take this requirement into account. I would be looking to keep checkpoint times under the application timeout the vast majority of the time. I have to add that the exact source of these delays (other than the fact there is a checkpoint in progress) and what are the optimal settings for LRUs, checkpoint interval etc. are not clear right now.

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    Benjamin Thompson
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  • 11.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    InnerCircle
    Posted Tue March 08, 2022 11:52 AM
    Hi Ben. It sounds like we need to work together to narrow down where the response time blip tends to happen. If it's after the checkpoint has truly started, then your theory about I/O is probably on target. There's an equal chance that it's happening before the checkpoint has a chance to start, when the checkpoint has been requested but it's waiting for threads to exit critical sections. As you probably know, during this window no other thread is allowed to enter a critical section, and although this wait time should be very short we have fixed defects in this area that allowed threads to linger there too long.

    onstat -g ckp does show the longest time a thread waited to enter a critical section before each checkpoint. Are you seeing anything concerning there? Can you give me a ballpark figure for the application timeout value? Is it seconds, tenths of seconds, hundredths?

    I'd be happy to get together with you and Jeff, either on the phone or over email, to narrow this down further if you like. If this is something that happens with any regularity there's a decent chance we can find the source and address it.

    Thanks.

    -jc

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    John Lengyel
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  • 12.  RE: Adjusting lru_min_dirty and lru_max_dirty when an instance is online

    InnerCircle
    Posted Tue March 08, 2022 06:07 PM
    Short comment.

    When investigating function "checkpointtime(lrumax,min)" make sure that actual number of modified buffers in the buffer pool is tracked. In some configurations Informix will not be able to maintain LRUMAX for one or more pools, that needs to be addressed on the storage / layout level.

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    Vladimir Kolobrodov
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