MySQL Bugs: #77957: search_latch_timeout doesn't make sense after supporting multi AHI latch

Bug #77957	search_latch_timeout doesn't make sense after supporting multi AHI latch
Submitted:	6 Aug 2015 9:31	Modified:	12 Aug 2015 17:16
Reporter:	zhai weixiang (OCA)	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S3 (Non-critical)
Version:	5.7.8	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
After supporting multi ahi latch in  MySQL 5.7.8,  btr_search_latch is always released in function row_search_mvcc if shortcut search is chosen.

grep the source code:

$grep 'search_latch_timeout' -R * --color
Binary file CMakeFiles/innobase.dir/row/row0sel.cc.gcno matches
handler/i_s.cc:                    (longlong) row->trx_search_latch_timeout, true));
include/trx0i_s.h:      ulint           trx_search_latch_timeout;
include/trx0i_s.h:                                      /*!< search_latch_timeout in trx_t */
include/trx0trx.h:      ulint           search_latch_timeout;
row/row0sel.cc:         trx->search_latch_timeout = BTR_SEA_TIMEOUT;
trx/trx0trx.cc: trx->search_latch_timeout = BTR_SEA_TIMEOUT;
trx/trx0i_s.cc: row->trx_search_latch_timeout = trx->search_latch_timeout;

Since we never hold a persistent btr_search_latch, I think the code bellow may be redundant

quoted from row_search_mvcc:

        if (trx->has_search_latch
#ifndef INNODB_RW_LOCKS_USE_ATOMICS
            && rw_lock_get_writer(
                btr_get_search_latch(index)) != RW_LOCK_NOT_LOCKED
#endif /* !INNODB_RW_LOCKS_USE_ATOMICS */
            ) {

                /* There is an x-latch request on the adaptive hash index:
                release the s-latch to reduce starvation and wait for
                BTR_SEA_TIMEOUT rounds before trying to keep it again over
                calls from MySQL */

                trx_search_latch_release_if_reserved(trx);

                trx->search_latch_timeout = BTR_SEA_TIMEOUT;
        }

How to repeat:
Read the code 

Suggested fix:
remove the redundant code.

Thank you for your report.

I have analyzed carefully the code that you are quoting and it does seem that there is some unnecessary code left for cleaning up.

Verified.

Related entry in 5.7.9 release notes:

"InnoDB: After disabling the adaptive hash index feature, an adaptive hash index latch was unnecessarily obtained and released . (Bug #21407023)"

Posted by developer:
 
Fixed as of the upcoming 5.7.9, 5.8.0 releases, and here's the changelog entry:

Code that provided a timeout mechanism intended to reduce adaptive hash
index search latch (btr_search_latch) contention was removed. The code
became obsolete after the introduction of adaptive hash index search
system partitioning in MySQL 5.7.8. 

Thank you for the bug report.

See also bug 79631