Tempdb latch contention
Posted by Karthick P.K on September 17, 2013
You might see Page latch contention in tempdb when you repeatedly drop and create TempDb objects (Temp tables, table variables etc.).
When you notice PAGELATCH_* contention on tempdb (Wait resource in sysprocesses starts with 2: ) check if the latch wait is on PFS,GAM or SGAM page. When there is latch contention on tempdb you will see lot of sessions waiting on Pagelatch_* similar to one below.
In the below output session is waiting on resource 2:15:121320 . If we decode the wait resource it is 2: database id of tempdb , 15: file number , 121320 is page number. 121320 is in multiple of 8088 so it is a PFS page, similarly identify if the page we are waiting is GAM or SGAM page if it is not PFS page.
Wait type Wait resource
How to identify if page is PFS,GAM or IAM?
PFS Page: A PFS page occurs once in 8088 pages. SQL Server will attempt to place a PFS page on the first page of every PFS interval(8088Pages). The only time a PFS page is not the first page in its interval is in the first interval for a file. File header page is first, and the PFS page is second. (Page ID starts from 0 so the first PFS page is at Page ID 1). If (page number)/8088 is round value then the page is PFS page.
GAM Page: GAM page is page 2 in the data file, next GAM page is placed at 511230 Page after first GAM page (GAM interval). If (page number-1)/511230 is round value then the page is GAM page.
SGAM Page: SGAM page is page 3 in data file , next SGAM page is placed at 511230 Page after first SGAM page. If (page number-2)/511230 is round value then the page is GAM page.
How to resolve?
1. Increase the number of TEMPDB data files files and size them equally. As a general rule, if the number of logical processors is less than or equal to 8, use the same number of data files as logical processors. If the number of logical processors is greater than 8, use 8 data files and then if contention continues further increase the number of data files by multiples of 4 (You may not see improvement once you reach 32 files).
2. Enable server side trace flag 1118.
3. If you further see latch contention on PFS page after following above two steps then the only option is to modify your application to limit the tempdb usage.
4. If you see contention on 2:1:103 (Page 103 is for system table sys.sysmultiobjrefs. This table manages the relationship between created objects in every database). The only way to reduce contention on this page is reduce the relation. Example creating lot of temp tables with primary key can cause this contention because the relation between the table and PK constraint has to be updated in sys.sysmultiobjrefs.
What’s the best practice ?
1. Create multiple tempdb data files instead of creating 1 large file and size them equally in all your SQL Server instances.
2. Make TF1118 (Uniform allocation) as default. (Extra space required by this trace flag shouldn’t really matter as amount additional space required is minimal and storage cost is not that high these days).
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