What is allocation bottleneck?

Managing TempDB in SQL Server: TempDB Configuration

SQL Server Storage Engine — Mon, 05 Jan 2009 02:51:42 GMT

In my previous blogs, I described the types of objects in TempDB and how they are managed. I hope that

TempDB Monitoring and Troubleshooting: Allocation Bottleneck

SQL Server Storage Engine — Sun, 11 Jan 2009 08:37:53 GMT

This blog continues the discussion on the common issues in TempDB that you may need to troubleshoot.

re: What is allocation bottleneck?

kirchner — Thu, 29 Jan 2009 13:21:16 GMT

Hi Sunil,

I've read many times SQL Server uses a deadlock-proof latching algorithm.

But by your description is certainly possible for a deadlock to occur.

If two processes are scanning a GAM page under SH latch, none of them will be able to take a X latch (latch compatibility rules are the same for locks???). Then if them both want to upgrade to a X latch, won't this be a deadlock?

re: What is allocation bottleneck?

Sunil Agarwal — Thu, 29 Jan 2009 20:51:45 GMT

Actually, the intent of my blog is to describe why latch contention can happen. This is what I had mentioned in the beginning of the BLOG. I checked with the person in the ;know' and he pointed outr that we take an UPDATE latch to flip the bit. I have updaed the blog.This does not block readers. I am sure the algorithm is more complex than I describe here for the reasons you outlined (i.e. avoid deadlock at all costs)

thanks for your interest and comments

re: What is allocation bottleneck?

kirchner — Thu, 29 Jan 2009 21:12:07 GMT

Hi,

I saw in the post your intention wasn't to describe the algorithm. I just have never been able to find something about HOW deadlock is actually avoided.

Thanks for the answer.