Why does an insert query occasionally take so long

Question

This is a pretty simple problem. Inserting data into the table normally works fine, except for a few times where the insert query takes a few seconds. (I am not trying to bulk insert data.) So I setup a simulation for the insert process to find out why the insert query occasionally takes more than 2 seconds to run. Joshua suggested that the index file may be being adjusted; I removed the id (primary key field), but the delay still happens.

I have a MyISAM table: daniel_test_insert (this table starts completely empty):

create table if not exists daniel_test_insert ( 
    id int unsigned auto_increment not null, 
    value_str varchar(255) not null default '', 
    value_int int unsigned default 0 not null, 
    primary key (id) 
)

I insert data into it, and sometimes a insert query takes > 2 seconds to run. There are no reads on this table - Only writes, in serial, by a single threaded program.

I ran the exact same query 100,000 times to find why the query occasionall takes a long time. So far, it appears to be a random occurrence.

This query for example took 4.194 seconds (a very long time for an insert):

Query: INSERT INTO daniel_test_insert SET value_int=12345, value_str='afjdaldjsf aljsdfl ajsdfljadfjalsdj fajd as f' - ran for 4.194 seconds
status               | duration | cpu_user  | cpu_system | context_voluntary | context_involuntary | page_faults_minor
starting             | 0.000042 | 0.000000  | 0.000000   | 0                 | 0                   | 0                
checking permissions | 0.000024 | 0.000000  | 0.000000   | 0                 | 0                   | 0                
Opening tables       | 0.000024 | 0.001000  | 0.000000   | 0                 | 0                   | 0                
System lock          | 0.000022 | 0.000000  | 0.000000   | 0                 | 0                   | 0                
Table lock           | 0.000020 | 0.000000  | 0.000000   | 0                 | 0                   | 0                
init                 | 0.000029 | 0.000000  | 0.000000   | 1                 | 0                   | 0                
update               | 4.067331 | 12.151152 | 5.298194   | 204894            | 18806               | 477995           
end                  | 0.000094 | 0.000000  | 0.000000   | 8                 | 0                   | 0                
query end            | 0.000033 | 0.000000  | 0.000000   | 1                 | 0                   | 0                
freeing items        | 0.000030 | 0.000000  | 0.000000   | 1                 | 0                   | 0                
closing tables       | 0.125736 | 0.278958  | 0.072989   | 4294              | 604                 | 2301             
logging slow query   | 0.000099 | 0.000000  | 0.000000   | 1                 | 0                   | 0                
logging slow query   | 0.000102 | 0.000000  | 0.000000   | 7                 | 0                   | 0                
cleaning up          | 0.000035 | 0.000000  | 0.000000   | 7                 | 0                   | 0

(This is an abbreviated version of the SHOW PROFILE command, I threw out the columns that were all zero.)

Now the update has an incredible number of context switches and minor page faults. Opened_Tables increases about 1 per 10 seconds on this database (not running out of table_cache space)

Stats:

• MySQL 5.0.89

• Hardware: 32 Gigs of ram / 8 cores @ 2.66GHz; raid 10 SCSI harddisks (SCSI II???)

• I have had the hard drives and raid controller queried: No errors are being reported. CPUs are about 50% idle.

• iostat -x 5 (reports less than 10% utilization for harddisks) top report load average about 10 for 1 minute (normal for our db machine)

• Swap space has 156k used (32 gigs of ram)

I'm at a loss to find out what is causing this performance lag. This does NOT happen on our low-load slaves, only on our high load master. This also happens with memory and innodb tables. Does anyone have any suggestions? (This is a production system, so nothing exotic!)

Answer 1

I had this problem using INNODB tables. (and INNODB indexes are even slower to rewrite than MYISAM)

I suppose you are doing multiple other queries on some other tables, so the problem would be that MySQL has to handle disk writes in files that get larger and needs to allocate additional space to those files.

If you use MYISAM tables I strongly suggest using

LOAD DATA INFILE 'file-on-disk' INTO TABLE `tablename` 

command; MYISAM is sensationally fast with this (even with primary keys) and the file can be formatted as csv and you can specify the column names (or you can put NULL as the value for the autoincrement field).

View MYSQL doc here.

Answer 2

The first Tip I would give you, is to disable the autocommit functionality and than commit manually.

LOCK TABLES a WRITE;
... DO INSERTS HERE
UNLOCK TABLES;

This benefits performance because the index buffer is flushed to disk only once, after all INSERT statements have completed. Normally, there would be as many index buffer flushes as there are INSERT statements.

But propably best you can do, and if that is possible in your application, you do a bulk insert with one single select.

This is done via Vector Binding and it's the fastest way you can go.

Instead
of:
"INSERT INTO tableName values()"
DO
"INSERT INTO tableName values(),(),(),().......(n) " ,

But consider this option only if parameter vector binding is possible with your mysql driver you're using.

Otherwise I would tend to the first possibility and LOCK the table for every 1000 inserts. Don't lock it for 100k inserts, because you'l get a buffer overflow.

Answer 3

By any chance is there an SSD drive in the server? Some SSD drives suffer from 'studder', which could cause your symptom.

In any case, I would try to find out if the delay is occurring in MySQL or in the disk subsystem.

What OS is your server, and what file system is the MySQL data on?

Answer 4

if you are using multiple insertion at one using for loop, then please take a break after every loop using PHP's sleep("time in seconds") function.

Answer 5

We upgraded to MySQL 5.1 and during this event the Query cache became an issue with a lot of "Freeing items?" thread states. We then removed the query cache.

Either the upgrade to MySQL 5.1 or removing the query cache resolved this issue.

FYI, to future readers.

-daniel

Answer 6

To check if your disk is behaving badly, and if you're in Windows, you can create a batch cmd file that creates 10,000 files:

@echo OFF
FOR /L %%G IN (1, 1, 10000) DO TIME /T > out%%G.txt

save it in a temp dir, like test.cmd

Enable command extensions running CMD with the /E:ON parameter

CMD.exe /E:ON

Then run your batch and see if the time between the first and the last out file differ in seconds or minutes.

On Unix/Linux you can write a similare shell script.