Reintroducing Random Readahead in InnoDB

Note: this article was originally published on http://blogs.innodb.com on July 25, 2011 by Inaam Rana.

Yes, we are reintroducing random readahead. We took the random readahead code out after our internal benchmarking and community feedback showed that random readahead tends to adversely affect the performance. However, some recent feedback from customers have shown that for some specific workloads random readahead can be good. Based on this information we have decided to reintroduce random readahead albeit with a dynamic switch to turn it on or off. By default, random readahead is not enabled. You can turn it on by setting innodb_random_read_ahead = TRUE.

The term ‘random readahead’ is a bit of an oxymoron. Readahead seems more related to sequential access pattern. To clarify this I’ll try to explain how InnoDB internally triggers a readahead request. Data pages in InnoDB are laid out on the disk in chunks of 64 pages called extents. The decision about readahead is essentially about whether it makes sense to read in the entire extent instead of reading in the pages one by one as and when they are requested by the user. There are two types of readahead mechanisms inside InnoDB. There are some obvious and some subtle differences between the two.

Linear readahead is the one that decides whether or not to read in the ‘next’ extent and the decision is made based not only on the number of pages of the ‘current’ extent present in the buffer pool but also the access pattern. The ‘current’ extent means the extent which is being accessed currently by the user threads. Whenever a user thread tries to access a page in the buffer pool the linear readahead mechanism is triggered. We first see if it is a boundary page of an extent or not. If it is a boundary page we figure out how many pages of this extent are present in the buffer pool and what is their access pattern. If there are more than innodb_read_ahead_threshold pages in the buffer pool and the access pattern is sequential and in the right order, InnoDB will queue asynchronous read requests for the entire next extent.

Random readahead is concerned mainly with the ‘current’ extent. The evaluation is done on each page read (note the difference from linear readahead where conditions are evaluated on every page access). On every page read, if innodb_random_read_ahead is set we try to see how many pages of this extent are in the buffer pool. If we have a certain number of pages and all of them have been accessed recently i.e.: they are closer to the MRU end of the LRU list we deduce that the remaining pages of the extent will be accessed soon as well. Asynchronous read requests for the remaining pages in the current extent are queued.

To cap the difference between the two types of readahead mechanisms, in case of linear readahead the decision is made about next extent, the conditions are evaluated on each page access, the access pattern is evaluated and decision is made if and only if we are accessing a boundary page of an extent. In case of random readahead the decision is made about current extent, the conditions are evaluated only at page read, instead of access pattern just the recentness of access is taken into consideration and the decision is not tied to the page being read in being a boundary page.

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