![]() > as we do currently but the double linked list is anchored in the hash cell allowing cheap Keep the bitmap - This change is a minor change, all we do is hash on Also, I can see that the bitmap was useful 10-15 years ago when memory >less common with the later versions as well. >Counteracting that is the fact that the default buffer pool is now 128M, so this would be Reclaimed from the memory heap of stale locks (just a thought). ![]() Perhaps the space lost by ditching the bitmap could be ![]() It is this that makes me suspicious of claims about the bitmap being the Was not that there were that many active locks but lots of stale locks on that transactions Where 256MB was used by single transaction for record locks. There are several causes for this too, I've debugged a case reported by internal testing >space required multiplier is large, it could certainly be more common, which isn't good. >Generally this has only occurred with people with very small buffer pools, but if the >InnoDB: transactions do not set too many row locks. >InnoDB: lock heaps or the adaptive hash index! Check that your >InnoDB: WARNING: over 4 / 5 of the buffer pool is occupied by >I've seen error messages from customers regarding the running out of memory for the lock The downside is that the idea will use more memory for at leastġ and 2 use cases above. INSERT.SELECT setting shared locks on the SELECT clause >locks, so those need to be thought of as well. However, there are some common scenarios which can set an enormous amount of >Generally speaking on most production systems there are thousands of locks at any given Haven't thought this through but some type of hybrid of the above two. Here by making the locking more fine grained and/or using lock free list for the actual locks.ģ. Remove the bitmap altogether and use as the key and anchor ![]() Current design only allows cheap forward traversal.Ģ. Still the expense of traversing all the locks on a page when we are only interested in the I tried this and it improves things somewhat but there is Keep the bitmap - This change is a minor change, all we do is hash on Īs we do currently but the double linked list is anchored in the hash cell allowing cheapįorward and backward traversal. >tree or something else? Assuming it isn't horrible, then it could certainly be useful ifġ. >What is the alternative to the bitmap? What sort of space multiplier would it be with a Generally this has only occurred with people with very small buffer pools, but if the space required multiplier is large, it could certainly be more common, which isn't good.Ĭounteracting that is the fact that the default buffer pool is now 128M, so this would be less common with the later versions as well. InnoDB: transactions do not set too many row locks. InnoDB: lock heaps or the adaptive hash index! Check that your InnoDB: WARNING: over 4 / 5 of the buffer pool is occupied by I've seen error messages from customers regarding the running out of memory for the lock table, such as: However, there are some common scenarios which can set an enormous amount of locks, so those need to be thought of as well. Generally speaking on most production systems there are thousands of locks at any given time. What is the alternative to the bitmap? What sort of space multiplier would it be with a tree or something else? Assuming it isn't horrible, then it could certainly be useful if it really is much faster. It is possible to hit this with regular binaries as well, but it is relatively rare since normally deadlock detection is hit first.įind a new algorithm to use for removing locks from the queue or new data structure which allows it to be more efficient. Once the deadlock detection is faster (or disabled) from BUG #49047, then the dequeueing becomes the prominent bottleneck. ![]() Run many threads doing many updates on a single row. Samples % image name app name symbol nameĢ57671 91.1329 mysqld mysqld lock_rec_has_to_wait_in_queueġ8913 6.6891 no-vmlinux no-vmlinux /no-vmlinuxġ214 0.4294 mysqld mysqld lock_rec_dequeue_from_page InnoDB loops through all locks that were blocking on the removed lock and then for each has to verify if each can be granted or not.Īnother big problem with this is that the kernel_mutex is held during the time, which restricts other operations when they conflict on the kernel_mutex.Ĭounted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask of 0x00 (Unhalted core cycles) count 100000 The problem is that removing a lock from queue is a O(N^2) operation. Under high load with many transactions pending on a few rows, it can be possible to run into very high CPU usage due to removing locks from queue. ![]()
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