Comments on: Wasting InnoDB memory

By: Mark Callaghan

Mark Callaghan — Sat, 31 May 2008 13:58:22 +0000

Domas,

I think the fix is to shrink the size and to not have an os_event_struct per rw-mutex/mutex. This should cut the memory consumption by more than 1/3 without major surgery.

By: John Sichi

John Sichi — Fri, 30 May 2008 20:51:23 +0000

I don’t know InnoDB internals, but in general, trying to pool RW locks for buffer pool pages can be problematic. If access methods are allowed to lock multiple pages simultaneously, then you can introduce spurious deadlocks that way.

For example, you might have access methods which are supposed to lock index pages in tree order (from root to leaf) so as to avoid deadlocks. Let’s say I’ve got a three-level tree, and the hash for one of the leaves is the same as the hash for the root. Thread 1 locks the root and then an intermediate page. Thread 2 locks the same intermediate page and then the leaf. Oops, deadlock.

If InnoDB doesn’t use the RW locks as medium-duration physical page locks like this (or if it can handle deadlocks at some level), then it’s not an issue.

Short-duration mutexes are easier.

JVS

By: Domas Mituzas

Domas Mituzas — Fri, 30 May 2008 06:39:13 +0000

Mark, I was looking at 64-bit builds, those are the ones that tend to have big buffer pool. But yeah, reducing structure sizes is another way to help here.

Peter, thats what my patch does =)

By: Peter Zaitsev

Peter Zaitsev — Fri, 30 May 2008 06:13:28 +0000

I think there is also other point – do we really need so MANY mutexes ?

One mutex is bad – millions is not good either. Why does not Innodb get to have some hashing for the mutexes to keep balance between contention and overhead.

By: Mark Callaghan

Mark Callaghan — Fri, 30 May 2008 01:17:00 +0000

My numbers leave out the size of the os_event_struct, which is 132 bytes on my 32-bit build. Each mutex_struct and rw_lock_struct instance will have 1 os_event_struct in the patch.

By: Mark Callaghan

Mark Callaghan — Fri, 30 May 2008 01:09:31 +0000

For a 32 bit build on Linux 2.6, sizeof(mutex_struct) is 104 and sizeof(rw_lock_struct) is 176. With our unreleased patch sizeof(mutex_struct) == 40 and sizeof(rw_lock_struct) = 112. I think that both can be reduced by another 10 bytes.

By: Mark Callaghan

Mark Callaghan — Fri, 30 May 2008 01:01:17 +0000

The structs (mutex_struct, rw_lock_struct and os_event_struct) can be made smaller, especially on 64-bit servers. Look for it in the next patch and keep the good ideas coming.

By: Kevin Burton

Kevin Burton — Fri, 30 May 2008 00:03:49 +0000

InnoDB is very elegant in some respects but totally broken in others.

Hopefully with more people looking at the code this will be fixed.

By: Aidas Kasparas

Aidas Kasparas — Thu, 29 May 2008 11:06:01 +0000

You did not forgot that your employer is a *hardware* company, no? ;-)

By: Arjen Lentz

Arjen Lentz — Thu, 29 May 2008 10:09:25 +0000

“enough memory” is not an excuse for bloat, though…