Often when troubleshooting performance related issues, it is beneficial to know when new indexes were introduced. However currently there is no easy way to keep track of this information or attain this information. That’s why one of my colleagues and a good friend, Mohamed Sharaf (Blog | Twitter), has submitted a request on Microsoft Connect, Expose index creation date in sys.indexes, Link. Please take a few minutes to vote if you agree .
Monthly Archives: October 2015
SQL Server Transaction Logs record the entire information about the transactions and data modifications made by the transactions in the server. In order to create backup or ensure the availability of transaction logs at the instance of any disaster, SQL offers two processes- SQL Log Shipping and SQL Log Mirroring.
In this section, we will describe both these processes and the parameters in which they differ from each other. Read the rest of this entry »
Final post in the series, I hope to summarize my findings for everyone. Thanks for all the comments, feedback and comments I have received to-date on these posts.
Before providing the summary of all the testing, research and reading, I want to provide some internals for folks.
NUAM Memory Allocation
If a server has NUMA configuration, when we define the max memory setting. All memory gets allocated evenly from all NUMA nodes. For example, if I allocate 8GB from a server that has 4 NUMA nodes, it will allocate 2GB from each NUMA node for the instance in question. So be careful, if you are using CPU affinity. As even using CPU affinity, lets say you only want to use NUMA 1 for an instance; this can lead to foreign memory access. NUMA allocation still happens across all NUMA nodes (at least in SQL Server 2014). So even though you might only have subset of schedulers online, memory will still be accessed across all NUMA Nodes. Even though with new hardware foreign memory access does not have major impact on performance. Still recommend avoiding it. If you must use CPU affinity, then allocate sub-set of the processor from each NUMA node.
Lazy Writer Responsibility
Lazy Writer is back ground process and a thread is created per physical NUMA node (for Soft-NUMA, we only get one thread, you can read details here by Jonathan Kehayias (Blog|Twitter)). Lazy Writer responsibility is to make sure sufficient number of free pages, about 640 pages, are always available for new data to be loaded. If Lazy Writer thread is spinning, that is usually a clear signal of internal memory pressure.
Note: Internal Memory pressure does not mean server doesn’t have enough memory. There are multiple factors that can contribute to this, one fact is low Max Server Memory, which is what has been explored in these articles.
Original Scenario: SQL Server 2012 Latest Build + Windows 2008 R2 Latest Build + Locked Pages In Memory + Low Max Server Memory + AMD Processors + NUMA Configuration
In this build, I can consistently reproduce the issue. If I set the Max Memory low enough, it caused the Lazy Writer on NUMA node 0 to spin non-stop, as I documented in SQL Server 2012 NUMA Node Imbalance – Cont’d. To further to this I understand some additional internals around this thanks to colleague who helped here. As I was not true sure what is happening.
If we looked at the DBCC MEMORYSTATUS, in particular we notice following for Node 0:
Memory node Id = 0 KB
VM Reserved 127086340
VM Committed 642452
Locked Pages Allocated 55996
Pages Allocated 54008
Pages Free 0
Target Committed 512000
Current Committed 698448
Foreign Committed 0
Away Committed 0
Taken Away Committed 0
Review the two numbers high-lighted above, Target memory is what this NUMA node is allocated and Current is what is NUMA consuming, therefore the Page Free count is 0. Thus Lazy Writer spinning to free up memory. However it is unable, thus spinning indefinitely causing CPU to pin. Question to answer, why can’t Lazy Writer free up memory from NUMA Node 0?
Looking further into MEMORYSTATUS, we also notice, that most of the memory allocated into NUMA Node 0 is for buffer cache:
MEMORYCLERK_SQLBUFFERPOOL (node 0) KB
VM Reserved 103971820
VM Committed 524288
Locked Pages Allocated 2524
SM Reserved 0
SM Committed 0
Pages Allocated 8
Test Scenario #1: Windows 10 + SQL Server 2012 RTM + Low Max Server Memory + Intel Processors + No-NUMA (Physical)
In this, I was just trying to reproduce the issue on my desktop, with no luck. Memory allocation or such were nominal.
Test Scenario #2: Windows 2012 + SQL Server 2014 RTM + Low Max Server Memory + Intel Processors + No-NUMA (Hyper-V)
Again no issue, as per the title of the series, NUMA Imbalance. It was obvious and expected this two scenarios don’t show any issues. But I wanted to confirm it.
Test Scenario #3: SQL Server 2008 R2 + Windows 2008 R2 + Low Max Server Memory + AMD Processors
These test was on same servers as original scenario, however we did not notice any issue.
*** During testing, we realized an interesting fact about NUMA configuration in SQL Server 2008 R2. SQL Server swaps NUMA 0 and NUMA 1 CPU mapping when creating a logical mapping for schedulers. For example, if you have 2 NUMA Nodes each with 4 cores; SQL Server binds NUMA 0 – Processor 0 – 3 to Scheduler 4 – 7 and NUMA 1 – Processor 4 – 7 to Scheduler 0 – 3. This is considered NUMA Swap, documented in this MSDN blog article here by CSS Team. However this behavior does not seem to persist in SQL Server 2012.
Test Scenario #4: SQL Server 2014 + Windows 2012 R2 + Low Max Server Memory + Intel Processors + NUMA (Azure IaaS Virtual Machine)
This was interesting case, when I set the Max Server Memory to low value, the NUMA node 0’s processor time started to increase. However it was not as swear as SQL Server 2012, however these are different processors. I had same behavior as noticed above, Current Committed was higher then Target Committed, therefore NUMA thread on Node 0 was spinning. However interesting thing note, it was not as aggressive as SQL Server 2012. It was not continuously spinning, it allowed CPU to breath. However would wake up frequently as low as ever 20ms.
!!! Eliminated Locked Pages in Memory as a factor for NUMA Imbalance.
Test Scenario #5: SQL Server 2014 + Windows 2012 R2 + Low Max Server Memory + Intel Processors + Locked Pages In Memory + NUMA (Azure IaaS Virtual Machine)
Exact same behavior as above was noticed, however, after restart, SQL Server no longer had Current Committed higher then Target Committed. Even after lowering the max memory even further then Scenario #4. It did not cause as dramatic shift.
*** Another interesting thing to note in SQL Server 2014. Up-to SQL Server 2012, Lazy Writer Thread bound to CPU 0 of each NUMA Node. In SQL Server 2014, Lazy Writer Thread is binding to last CPU of each NUMA Node.
Conclusion & Things to Consider
- Configuring SQL Server Max Server Memory low memory will cause Lazy Writer to spin due to internal memory pressure issue.
- There is no value at which point this behavior starts, it depends on the load of the server. For example, in Original Scenario, due to server load under 4000MB it started spinning the Lazy Writer thread; however in Test Scenario #4, I had to go down to as much as 1000MB. Therefore on your servers look at Lazy Writer thread in conjunction with CPU and DBCC MEMORYSTATUS to understand what is truly happening.
- This behavior is consistent in whether I have Locked Pages in Memory or not.
- In Windows 2012 R2 NUMA allocation seem to be much better then Windows 2008 R2.
- SQL Server 2014 Lazy Writer thread does not seem to be as aggressive as SQL Server 2012.
- Few KB articles came up during my research in SQL Server 2012 that fix NUMA related issues, KB2819662 and KB2926223.
- There as been some fixes released for NUMA for Windows 2008 R2 operating system also, please review KB2155311 and KB2510206.
- In addition, if you are running a server with more then 64-logical cores, please review K-Group Configuration, for details please reference this article.
- Last but not least verify that memory is distributed evenly on physical server. That is the memory banks have been evenly allocated, i.e. you do not have more memory allocated in one bank versus another, as you might also see one NUMA node working extra harder if there is significant amount of foreign memory access.
Question to answer, why can’t Lazy Writer free up memory from NUMA Node 0?
SQL Server Lazy Writer is only responsible for clearing memory from Buffer Pool, it cannot remove or clear memory from any other memory clerk. Therefore, it is possible, that other internal components have memory allocated on NUMA Node 0 only, therefore NUMA Node 0 is unable to free up enough memory for buffer pool only. Looking at DBCC MEMORYSTATUS we can see which memory clerks allocate from NODE Node 0 only, there are few, for example, MEMORYCLERK_SQLCLR, MEMORYCLERK_SOSMEMMANAGER, OBJECTSTORE_LOCK_MANAGER, and MEMORYCLERK_XE_BUFFER all have allocation from NODE Node 0 only.
In our previous discussion, we have discussed about transaction log corruption and backup in SQL Server. Now, in this article we will know the steps for creating system stored procedure in SQL Server 2008 R2.
Organizations where SQL Server system is used for database storage and maintenance, projects with 1000s of database tables and stored procedures are common to find. Adding even a single parameter to an existing stored procedure can straightaway put an impact on a large number of other stored procedures or database tables as they are called from them. Moreover, Read the rest of this entry »
Yesterday, I had posted an issue I been troubleshooting with memory allocation. That was causing CPU to pin, you can read more about the issue at SQL Server 2012 NUMA Node Imbalance. Even though I found what was causing it, I didn’t understand the internals well enough to identify the root cause.
Additional testing I discovered this issue is repro on Windows 2008 R2 Server with SQL Server 2012. It does not seem to fixed that at 4000MB the issue will start; if server is busy, then it can start at a higher value. In my re-pro test, I found it starting at even 6000MB.
As I said the up front memory allocation and management was contributing to this, however now I am unsure if Locked Pages in Memory is to contribute to this.
However, further insight I learned today. If this issue exists, you might have Lazy Writer spinning continuously on the effected NUMA node.
Therefore if you execute following T-SQL:
select session_id, wait_type, wait_time, cpu_time, scheduler_id
where command = ‘lazy writer’
order by scheduler_id
Look at the output generated, if your Lazy Writer thread is working as normal (aka, only wakes up sometimes) you should see something similar to following:
However if you have this issue the Lazy Writer might look like below:
- Notice the wait_type has gone to NULL.
- Notice wait_time has gone to 0 – signal it is not going to sleep like the rest.
- Notice cpu_time keeps climbing.
Only work around that I know for this currently is, increase the max memory until you see lazy writer stop spinning and processor utilization starts dropping.
One of my client has been fighting a very strange problem and there is not much literature on the topic, NUMA Node Imbalance.
- 8 NUMA nodes with 6 cores each; totaling 48 cores.
- 256GB memory.
- 12 Instances of SQL Server; mix of SQL Sever 2012 Latest Build and SQL Server 2008 R2 SP2.
- NUMA node 0 pinned to 100% when all 12 instances are set to automatic CPU affinity setting. This is recommended as default, there have been very few cases where I have had to consider changing this. Generally default is fine, the SQL Server internal engine (or SQLOS)’s scheduler handle is very efficient at distributing load. So why do we have it that NUMA node 0 was pinned?
- Initial Troubleshooting
Any time we have performance problem on a server that is running SQL Sever instances; my first question to ask is, is it really SQL Sever? Looking at task manager or resource governor, both show highest CPU utilization of under 5%. This is not surprising, as the CPU in both of these tools provides an average over all CPUs. Therefore there is no easy way to know which instances were causing the NUMA Node 0 on the server to pin. There is nothing obvious that is also visible if I filter through the DMV/DMFs with in each instance.
However due to performance issues, my client decided to manually distribute the load across the 8 NUMA nodes by using CPU affinity mask setting. And sure enough load on NUMA node 0 went down; however he only did it for subset of the instances. So load decreased from 100% to about 70%.
Research and Investigation
While doing investigation, I only found two KB articles that talk about NUMA node performance issue on SQL Server 2012, KB2819662 and KB2926223. However, all the SQL Server 2012 instances are on latest build; therefore, these hot fixes did not apply. I also found KB relating to operating system and NUMA processor pressure issues, however all turned out to be not the issue here. Because even operating system was fully patched.
In order to troubleshoot what is causing CPU pressure issue on NUMA node 0, we decided to move all instances off NUMA node 0 by using CPU affinity mask setting. In doing so we eventually found instances that was causing the load. There were 6 instances that were contributing the load.
After investigating we found, locked pages in memory is enabled by default on each instance and found all affected instances had their maximum set to 4000MB. I remember reading it is no recommended to have maximum memory set to such a low value. Therefore, we increased it to 6000MB. Doing so instantly caused the CPU on NUMA node 0 to return to nominal levels.
Memory settings on SQLInstance01
Processor Affinity Mask Setting – Set to NumaNode03 for SQLInstance01
Above is an example of SQLInstance01, set to 4000MB with processor affinity set to NumaNode03. If I look at the CPU now, there is no issues as in screenshot below. However load is not running on NUMA node 0 right now.
CPU Load on NUMA Node 0 (highlighted in red) – Nominal
Processor Affinity Mask Setting – Set to Default for SQLInstance01
Now if I change the instance to use all NUMA nodes, as per the default configuration, as in screenshot above. Processors on NUMA node 0 start to pick up again. Notice the change was almost instantaneously. Since I only did it for one instance it did not cause processor to pin to 100%; however hovered around 40% constant. Imagine that times 6 instances, no wonder NUMA node 0 was pinned to 100%.
CPU Load on NUMA Node 0 (highlighted in red) – After Affinity Set to Auto in SQL Server
Now if I adjust the max memory setting to 8000MB (I tested it with 6000MB); both settings, processor returned to normal performance; as per the screenshots below.
Memory setting on SQLInstance01 – Setting it to 8000MB
CPU Load on NUMA Node 0 (highlighted in red) – After setting the Memory to > 4000MB
If you are using 64-bit of SQL Server (which is all version now) and have locked pages in memory enabled. Be careful in setting the maximum memory value to a low value. Because of locked pages in memory additional work needs to be done by SQL Server to manage the memory and from looks of it all this work is happening on NUMA node 0. I do not know if this is by design. However setting the memory to value higher then or equal to 6000MB resolved this issue.
I’ll follow up on this if I have any updates, as I’ll be raising the issue internally also.