MySQL Bugs: #57707: InnoDB buf_page_t size has doubled on 32-bit systems

Bug #57707	InnoDB buf_page_t size has doubled on 32-bit systems
Submitted:	25 Oct 2010 8:30	Modified:	9 Dec 2010 22:40
Reporter:	Marko Mäkelä	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S3 (Non-critical)
Version:	5.5+	OS:	Any
Assigned to:	Marko Mäkelä	CPU Architecture:	Any
Tags:	buf_page_t, compression, innodb_buffer_pool_instances

Description:
When multiple buffer pools (innodb_buffer_pool_instances) were implemented in InnoDB, a buf_pool pointer was added to buf_page_t. Before this change, the storage size of buf_page_t was exactly 64 bytes on 32-bit systems, which avoided any memory fragmentation in buf0buddy.c. After this change, the memory use of small-page-size compressed tables increased significantly.

I noticed this, because code built with UNIV_DEBUG_VALGRIND complained about uninitialized bytes in buf_page_t. These checks are only enabled on 32-bit systems, because on 64-bit systems the pointers in buf_page_t would make the structure larger, containing some uninitialized pad bytes.

How to repeat:
Build 32-bit binaries of MySQL 5.5 or later. Note that sizeof(buf_page_t) is more than 64 bytes. Alternatively, build it with Valgrind support enabled, and observe the warnings about uninitialized bytes during custom check requests.

Suggested fix:
Remove the buf_pool pointer from buf_page_t. Use a 6-bit index instead (enough for the current maximum of 64 buffer pools). Allocate the 6 bits by reducing the size of buf_fix_count from 25 to 19 bits (524,288 concurrent transactions should still be more than enough).

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/122020

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/122021

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/122138

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/122139

Pushed into mysql-trunk 5.6.99-m5 (revid:alexander.nozdrin@oracle.com-20101113155825-czmva9kg4n31anmu) (version source revid:alexander.nozdrin@oracle.com-20101113152450-2zzcm50e7i4j35v7) (merge vers: 5.6.1-m4) (pib:21)

Pushed into mysql-next-mr (revid:alexander.nozdrin@oracle.com-20101113160336-atmtmfb3mzm4pz4i) (version source revid:alexander.nozdrin@oracle.com-20101113152540-gxro4g0v29l27f5x) (pib:21)

Pushed into mysql-5.5 5.5.9 (revid:jonathan.perkin@oracle.com-20101216101358-fyzr1epq95a3yett) (version source revid:jonathan.perkin@oracle.com-20101216101358-fyzr1epq95a3yett) (merge vers: 5.5.9) (pib:24)