Maximum possible memory usage: 16.2G (874% of installed RAM) Why the MySQL is saying that can use about 16GB of RAM even if my server has only about 2GB?

# ./mysqltuner.pl

 &gt;&gt;  MySQLTuner 1.2.0 - Major Hayden &lt;major@mhtx.net&gt;
 &gt;&gt;  Bug reports, feature requests, and downloads at http://mysqltuner.com/
 &gt;&gt;  Run with '--help' for additional options and output filtering

-------- General Statistics --------------------------------------------------
[--] Skipped version check for MySQLTuner script
[OK] Currently running supported MySQL version 5.5.34-cll
[OK] Operating on 32-bit architecture with less than 2GB RAM

-------- Storage Engine Statistics -------------------------------------------
[--] Status: +Archive -BDB -Federated +InnoDB -ISAM -NDBCluster
[--] Data in MyISAM tables: 457M (Tables: 1361)
[--] Data in MRG_MYISAM tables: 53M (Tables: 36)
[--] Data in InnoDB tables: 792M (Tables: 1133)
[--] Data in PERFORMANCE_SCHEMA tables: 0B (Tables: 17)
[--] Data in MEMORY tables: 0B (Tables: 2)
[!!] Total fragmented tables: 175

-------- Security Recommendations  -------------------------------------------
[OK] All database users have passwords assigned

-------- Performance Metrics -------------------------------------------------
[--] Up for: 54s (2K q [37.537 qps], 325 conn, TX: 1M, RX: 354K)
[--] Reads / Writes: 90% / 10%
[--] Total buffers: 1.7G global + 18.6M per thread (800 max threads)
[!!] Allocating &gt; 2GB RAM on 32-bit systems can cause system instability
[!!] Maximum possible memory usage: 16.2G (874% of installed RAM)
[OK] Slow queries: 0% (9/2K)
[OK] Highest usage of available connections: 1% (13/800)
[OK] Key buffer size / total MyISAM indexes: 128.0M/78.9M
[!!] Key buffer hit rate: 73.9% (1K cached / 297 reads)
[OK] Query cache efficiency: 42.4% (388 cached / 915 selects)
[OK] Query cache prunes per day: 0
[OK] Sorts requiring temporary tables: 0% (0 temp sorts / 148 sorts)
[OK] Temporary tables created on disk: 21% (114 on disk / 537 total)
[OK] Thread cache hit rate: 94% (17 created / 325 connections)
[OK] Table cache hit rate: 99% (2K open / 2K opened)
[OK] Open file limit used: 1% (2K/200K)
[OK] Table locks acquired immediately: 100% (1K immediate / 1K locks)
[OK] InnoDB data size / buffer pool: 792.7M/1.0G

-------- Recommendations -----------------------------------------------------
General recommendations:
    Run OPTIMIZE TABLE to defragment tables for better performance
    MySQL started within last 24 hours - recommendations may be inaccurate
    Enable the slow query log to troubleshoot bad queries


And buffer related settings

mysql&gt; SHOW VARIABLES LIKE '%buffer%';
+------------------------------+------------+
| Variable_name                | Value      |
+------------------------------+------------+
| bulk_insert_buffer_size      | 8388608    |
| innodb_buffer_pool_instances | 1          |
| innodb_buffer_pool_size      | 1073741824 |
| innodb_change_buffering      | all        |
| innodb_log_buffer_size       | 8388608    |
| join_buffer_size             | 16777216   |
| key_buffer_size              | 134217728  |
| myisam_sort_buffer_size      | 8388608    |
| net_buffer_length            | 16384      |
| preload_buffer_size          | 32768      |
| read_buffer_size             | 131072     |
| read_rnd_buffer_size         | 262144     |
| sort_buffer_size             | 2097152    |
| sql_buffer_result            | OFF        |
+------------------------------+------------+
14 rows in set (0.00 sec)
Staring at the mysqltuner output, I see something disturbing
[--] Total buffers: 1.7G global + 18.6M per thread (800 max threads)


The 800 is the max_connections.
The 18M is a sum that includes read_buffer_size + sort_buffer_size + join_buffer_size

What is 800 * 18M ?
mysql&gt; select 18 * 1024 * 1024 * 800 / power(1024,3);
+----------------------------------------+
| 18 * 1024 * 1024 * 800 / power(1024,3) |
+----------------------------------------+
|                                14.0625 |
+----------------------------------------+
1 row in set (0.00 sec)

That's 14G your DB Server cannot handle.
RECOMMENDATION
Add this to my.cnf
[mysqld]
sort_buffer_size = 1M
join_buffer_size = 1M
read_buffer_size = 1M
max_connections = 100

then restart mysql and run mysqltuner.pl again. You will see lower numbers.
Give it a Try !!!