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Re: MySQL gets long semaphore locks on dict0dict.cc - but on DML operations
Posted by: Nikhil Nair
Date: August 22, 2016 11:32AM

Its queueing because the level of concurrency is too high for the number of processes. Plus I need a queue to maintain order and such.

I have not checked select count (*). I know for sure there are rows, but it might not be 17k. This is from the SQL pro window of approx rows - which I think comes from Information_schema.

It is with innodb_file_per_table=ON. I considered running optimize table - but in the event that it is not instantaneous, I'd hit massive backlog.

The 99GB I suspect is because rows weren't deleted. This table has been operational for around a year.

Compressing is certainly possible, I could give that a shot - but do you suspect that its the time for the application to retrieve the data thats causing the locks, or just its contribution to size?

I appreciate the discussion - i've run tests on non-production machines - with a watered down instances (lower buffer pool, smaller disk size, fewer CPU etc) and smaller load, while I see numerous locks there as well (chiefly when the table cannot fit in buffer_pool size), i cannot pin it down to this particular problem.

As a solution, I'm changing the application a little - to not rely on a single table, but distributing the payload. So the queries will not hit only 1 table, it'll hit different ones.

I still don't know what's causing this particular sem lock.

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Re: MySQL gets long semaphore locks on dict0dict.cc - but on DML operations
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