Comments on: The mysterious world of shmmax and shmall http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/ News and views from Pythian DBAs Fri, 10 Feb 2012 13:01:25 +0000 hourly 1 http://wordpress.org/?v=3.0.4 By: What is shmmax, shmall, shmmni? Shared Memory Max | Useful Stuff. http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-629167 What is shmmax, shmall, shmmni? Shared Memory Max | Useful Stuff. Fri, 04 Nov 2011 05:30:03 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-629167 [...] mysterious setting has been explained pretty well here: and am reposting my findings since last time I dealt with these settings, I just solved my problem [...] [...] mysterious setting has been explained pretty well here: and am reposting my findings since last time I dealt with these settings, I just solved my problem [...]

]]> By: Setting shmmax, shmall oracle « Just Do It. Now. http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-507755 Setting shmmax, shmall oracle « Just Do It. Now. Fri, 11 Feb 2011 08:27:17 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-507755 [...] The mysterious world of shmmax and shmall Over the Top Tales from the trenches. Motto: Bringing order to the chaos of every day DBA life. [...] [...] The mysterious world of shmmax and shmall Over the Top Tales from the trenches. Motto: Bringing order to the chaos of every day DBA life. [...]

]]> By: Robert Dupuy http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-412885 Robert Dupuy Wed, 10 Mar 2010 02:18:48 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-412885 See this "oracle validated configuration" here: http://linux.oracle.com/pls/apex/f?p=102:2:4310916404894033::NO::P2_VC_ID:471 See this “oracle validated configuration” here:

http://linux.oracle.com/pls/apex/f?p=102:2:4310916404894033::NO::P2_VC_ID:471

]]> By: Robert Dupuy http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-412883 Robert Dupuy Wed, 10 Mar 2010 02:16:30 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-412883 Oracle has a certified system that has 8GB of physical RAM and 4TB SHMALL setting. So, it is not, necessarily a 'recipe for paging hell'. From what I understand, it's just an upper limit. Using too much RAM, is a recipe for paging hell, but what do you want to happen, the system to crawl to a halt, or the system to completely stop working with errors. Setting the limit lower, just means you will have the out of memory error. The long term solution seems to be, don't use more memory than you need. With that said, I have had all kinds of problems trying to set a large SGA without getting herky-jerky results on Linux. But setting this limit lower as desribed in this article, did not change the results for me, and I suspect it isn't the answer. Oracle has a certified system that has 8GB of physical RAM and 4TB SHMALL setting.

So, it is not, necessarily a ‘recipe for paging hell’. From what I understand, it’s just an upper limit.

Using too much RAM, is a recipe for paging hell, but what do you want to happen, the system to crawl to a halt, or the system to completely stop working with errors. Setting the limit lower, just means you will have the out of memory error.

The long term solution seems to be, don’t use more memory than you need.

With that said, I have had all kinds of problems trying to set a large SGA without getting herky-jerky results on Linux.

But setting this limit lower as desribed in this article, did not change the results for me, and I suspect it isn’t the answer.

]]> By: Don Turnblade http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-404245 Don Turnblade Tue, 09 Feb 2010 23:13:47 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-404245 Revisiting this question. The original author left 4 GB of RAM for Linux and Cache. Thus, Total RAM - 4 GB RAM = SHMMAX # As a percent of Physical Memory that is 100% * (1-4GB/Total) And, SHMALL = SHMAX / PAGE_SIZE So, for a 64 GB RAM, 64 bit system... That is 64 GB - 4 GB RAM = 60 GB SHMMAX ANd, SHMALL = 60 GB / 4096 = 15 MB SHMALL Any more than that cause paging hell as the Linux Kernel and Cache become involved in potential paging... If I read this all right, the only adjustment is to make sure that the Linux kernal and cache fit inside that 4 GB. If not make it larger and recompute. If I have this right, then the basic reason for loading Physical RAM in the first place -- reducing the opportunity for paging to occur is met and maximized. Did I get it right? Revisiting this question. The original author left 4 GB of RAM for Linux and Cache.

Thus, Total RAM – 4 GB RAM = SHMMAX
# As a percent of Physical Memory that is 100% * (1-4GB/Total)
And, SHMALL = SHMAX / PAGE_SIZE

So, for a 64 GB RAM, 64 bit system…
That is 64 GB – 4 GB RAM = 60 GB SHMMAX
ANd, SHMALL = 60 GB / 4096 = 15 MB SHMALL

Any more than that cause paging hell as the Linux Kernel and Cache become involved in potential paging…

If I read this all right, the only adjustment is to make sure that the Linux kernal and cache fit inside that 4 GB. If not make it larger and recompute.

If I have this right, then the basic reason for loading Physical RAM in the first place — reducing the opportunity for paging to occur is met and maximized. Did I get it right?

]]> By: Robby http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-399489 Robby Thu, 21 Jan 2010 10:03:29 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-399489 Dear Paul, so, from the articel above, we summarized: server 2 Quad Xeon, 16GB memory shmmax = 17179869184 (16GB) more than this = hell paging maximum amount of memory = shmall * pagesize 16GB = shmall * 4096 (default) shmall = 16GB / 4096 = 4194304 (4MB) is this right? "Making shmall larger than free RAM is a recipe for paging hell and much gnashing of teeth." -> means shmall can be set to max 17179869184? more than that, hell paging? "Oracle recommends half the RAM, we pushed the envelope and chose 75% as 8 gigabytes of free for OS and cache is just wasteful." -> recommends shmall or shmmax? if it's shmmax, why from the formula above i've got shmall=4MB even2 farrrrrrrrrrrrrrrrrrrrr from half of memory (8GB) from Oracle 10g Release 2 Linux x86 documentation shmmax = Half the size of physical memory (in bytes) Regards, Robby Junior DBA Dear Paul,

so, from the articel above, we summarized:
server 2 Quad Xeon, 16GB memory

shmmax = 17179869184 (16GB) more than this = hell paging

maximum amount of memory = shmall * pagesize
16GB = shmall * 4096 (default)
shmall = 16GB / 4096 = 4194304 (4MB)

is this right?

“Making shmall larger than free RAM is a recipe for paging hell and much gnashing of teeth.” -> means shmall can be set to max 17179869184? more than that, hell paging?

“Oracle recommends half the RAM, we pushed the envelope and chose 75% as 8 gigabytes of free for OS and cache is just wasteful.” -> recommends shmall or shmmax? if it’s shmmax, why from the formula above i’ve got shmall=4MB even2 farrrrrrrrrrrrrrrrrrrrr from half of memory (8GB)

from Oracle 10g Release 2 Linux x86 documentation
shmmax = Half the size of physical memory (in bytes)

Regards,

Robby
Junior DBA

]]> By: George http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-392527 George Mon, 14 Dec 2009 16:48:03 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-392527 not 34 G, 32 G of RAM not 34 G, 32 G of RAM

]]> By: George http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-392525 George Mon, 14 Dec 2009 16:47:13 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-392525 what is potential harm of having shmmax too high? i noticed on my RAC system i have shmmax = 21474836480 while oracle recommends "(4 GB - 1 byte), or half the size of physical memory (in bytes), whichever is lower" - we have 34 G RAM. It is 11g and we switched to using hugepages, previously used AMM - upon oracle support recommendation. thanks, --george what is potential harm of having shmmax too high? i noticed on my RAC system i have shmmax = 21474836480 while oracle recommends
“(4 GB – 1 byte), or half the size of physical memory (in bytes), whichever is lower” – we have 34 G RAM. It is 11g and we switched to using hugepages, previously used AMM – upon oracle support recommendation.

thanks,
–george

]]> By: A Chess http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-378805 A Chess Sun, 11 Oct 2009 07:00:53 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-378805 Very well written useful information. Thanks much! Very well written useful information.
Thanks much!

]]> By: Lukas http://www.pythian.com/news/245/the-mysterious-world-of-shmmax-and-shmall/#comment-366109 Lukas Mon, 13 Jul 2009 10:14:38 +0000 http://www.pythian.com/blogs/245/the-mysterious-world-of-shmmax-and-shmall#comment-366109 Just had an interesting query from a client who is trying to increase SGA + PGA memory beyond 12GB on a system with 32GB. If he adds +1GB to any area, the instance is not starting up. Not only it's not starting, but it's also not giving any hints, errors or other clues what's wrong. Alert log just says "Starting ORACLE instance (normal)" and nothing more (it's 10.2.0.3 RAC). It boiled down to the possibility that shmall is too small (3279547 ~ 12.5G). Thanks for the hint Paul! Just had an interesting query from a client who is trying to increase SGA + PGA memory beyond 12GB on a system with 32GB. If he adds +1GB to any area, the instance is not starting up. Not only it’s not starting, but it’s also not giving any hints, errors or other clues what’s wrong. Alert log just says “Starting ORACLE instance (normal)” and nothing more (it’s 10.2.0.3 RAC).

It boiled down to the possibility that shmall is too small (3279547 ~ 12.5G).

Thanks for the hint Paul!

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