Tempdb--Allocation Bottleneck

Alloctaion bottleneck refers to contention in the system pages that store allocation structures.

PFS(Page Free Space): used to track the followint information about each page:

1. free space available

2. if it is allocated or not

3. has ghost records(when a row is deleted, it is marked as ghost)

 

PFS use 1byte/page and each PFS page stores stats for 8088 pages

 

GAM(Global allocation Map): used  to trackis a uniform extent is free or not, use one bit to ideitify one extent, so each GAM page stores status for  64000 extents or 4 GB

 

SGAM(Share Allocation Map): tracks if an extent is mixed extent or not, one bit '1' indicates that it is a mixed extent and has one or more free pages, each SGAM page cover 64000 extent or 4GB

 

if one objects use more then 7 pages, SQL SERVER will allocate uniform extents for it.

 

Each database file have thire owm PFS/GAM/SGAM pages.

 

When one object was created(destoryed) or page was allocated(deallocated), SQL Server need search the allcation page with SH latch and update the allocation page with X latch, multiple thread need to wait to acquire X or SH latch in non coflicting mode which can lead to allocation bootleneck.

 

 

starting with sql server 2005, the cahcing mechanism for objects in TempDB Has been improved siginificantly which reduce the alloctaion contention incurred by your workload.

 

检查是否有分配页阻塞

--=============================================
--检查Tempdb上的页面争抢
--http://www.sqlsoldier.com/wp/sqlserver/breakingdowntempdbcontentionpart2
With Tasks
As (Select session_id,
        wait_type,
        wait_duration_ms,
        blocking_session_id,
        resource_description,
        PageID = Cast(Right(resource_description, Len(resource_description)
                - Charindex(':', resource_description, 3)) As Int)
    From sys.dm_os_waiting_tasks
    Where wait_type Like 'PAGE%LATCH_%'
    And resource_description Like '2:%')
Select session_id,
        wait_type,
        wait_duration_ms,
        blocking_session_id,
        resource_description,
    ResourceType = Case
        When PageID = 1 Or PageID % 8088 = 0 Then 'Is PFS Page'
        When PageID = 2 Or PageID % 511232 = 0 Then 'Is GAM Page'
        When PageID = 3 Or (PageID - 1) % 511232 = 0 Then 'Is SGAM Page'
        Else 'Is Not PFS, GAM, or SGAM page'
    End
From Tasks;