Reduced contention during datafile extension

Wednesday, July 27, 2011

Another performance problem found by PoorMansProfiler

Innam rana said in his blog post on innodb blog:

InnoDB has an internal file system management module that primarily manages the space in the data files. One of the pain points was the coarse level of locking used when a data file has to be extended. More about this issue can be found here. In the latest labs release we have fixed this problem.
When we need to extend a data file inside InnoDB we write zero filled pages synchronously to the file. The user thread which is extending the data file holds fil_system::mutex during the whole operation. This mutex covers changes to all data structures related to file system management. What this means is that when we do a regular IO we do need to acquire fil_system::mutex though only for a short time. Because the thread doing the data file extension is holding the mutex during the whole IO operation any other thread (user or background) trying to access data file for regular read or write ends up waiting. This brings the whole system to a virtual stand still as no read or write activity can happen. This is true even if a thread is trying to access a data file that is not the one being extended.
We fixed this issue by introducing an internal flag to the data structure indicating that a file is being extended. Now if a user thread needs to extend a data file it does acquire the fil_system::mutex but releases it after setting the flag. Once it is done with the extension IO it resets the flag. This allows other threads to access data files while one of the file is being extended. This also allows multiple files to be extended in parallel. Our tests have shown that the issue of stalls due to file extension is indeed fixed by this approach.
A related feature which can be considered as future work is to off load the file extension to a background thread.

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