I have a busy database with solely InnoDB tables which is about 5GB in size. The database runs on a Debian server using SSD disks and I've set max connections = 800 which sometimes saturate and grind the server to halt. The average query per second is about 2.5K. So I need to optimize memory usage to make room for maximum possible connections. 

I've seen suggestions that innodb_buffer_pool_size should be up to %80 of the total memory. On the other hand I get this warning from tuning-primer script:

Max Memory Ever Allocated : 91.97 G
Configured Max Per-thread Buffers : 72.02 G
Configured Max Global Buffers : 19.86 G
Configured Max Memory Limit : 91.88 G
Physical Memory : 94.58 G


Here are my current innodb variables:

| innodb_adaptive_flushing                          | ON                                                                                                                     |
| innodb_adaptive_hash_index                        | ON                                                                                                                     |
| innodb_additional_mem_pool_size                   | 20971520                                                                                                               |
| innodb_autoextend_increment                       | 8                                                                                                                      |
| innodb_autoinc_lock_mode                          | 1                                                                                                                      |
| innodb_buffer_pool_instances                      | 1                                                                                                                      |
| innodb_buffer_pool_size                           | 20971520000                                                                                                            |
| innodb_change_buffering                           | all                                                                                                                    |
| innodb_checksums                                  | ON                                                                                                                     |
| innodb_commit_concurrency                         | 0                                                                                                                      |
| innodb_concurrency_tickets                        | 500                                                                                                                    |
| innodb_data_file_path                             | ibdata1:10M:autoextend                                                                                                 |
| innodb_data_home_dir                              |                                                                                                                        |
| innodb_doublewrite                                | ON                                                                                                                     |
| innodb_fast_shutdown                              | 1                                                                                                                      |
| innodb_file_format                                | Antelope                                                                                                               |
| innodb_file_format_check                          | ON                                                                                                                     |
| innodb_file_format_max                            | Antelope                                                                                                               |
| innodb_file_per_table                             | ON                                                                                                                     |
| innodb_flush_log_at_trx_commit                    | 2                                                                                                                      |
| innodb_flush_method                               | O_DIRECT                                                                                                               |
| innodb_force_load_corrupted                       | OFF                                                                                                                    |
| innodb_force_recovery                             | 0                                                                                                                      |
| innodb_io_capacity                                | 200                                                                                                                    |
| innodb_large_prefix                               | OFF                                                                                                                    |
| innodb_lock_wait_timeout                          | 50                                                                                                                     |
| innodb_locks_unsafe_for_binlog                    | OFF                                                                                                                    |
| innodb_log_buffer_size                            | 4194304                                                                                                                |
| innodb_log_file_size                              | 524288000                                                                                                              |
| innodb_log_files_in_group                         | 2                                                                                                                      |
| innodb_log_group_home_dir                         | ./                                                                                                                     |
| innodb_max_dirty_pages_pct                        | 75                                                                                                                     |
| innodb_max_purge_lag                              | 0                                                                                                                      |
| innodb_mirrored_log_groups                        | 1                                                                                                                      |
| innodb_old_blocks_pct                             | 37                                                                                                                     |
| innodb_old_blocks_time                            | 0                                                                                                                      |
| innodb_open_files                                 | 300                                                                                                                    |
| innodb_purge_batch_size                           | 20                                                                                                                     |
| innodb_purge_threads                              | 0                                                                                                                      |
| innodb_random_read_ahead                          | OFF                                                                                                                    |
| innodb_read_ahead_threshold                       | 56                                                                                                                     |
| innodb_read_io_threads                            | 4                                                                                                                      |
| innodb_replication_delay                          | 0                                                                                                                      |
| innodb_rollback_on_timeout                        | OFF                                                                                                                    |
| innodb_rollback_segments                          | 128                                                                                                                    |
| innodb_spin_wait_delay                            | 6                                                                                                                      |
| innodb_stats_method                               | nulls_equal                                                                                                            |
| innodb_stats_on_metadata                          | ON                                                                                                                     |
| innodb_stats_sample_pages                         | 8                                                                                                                      |
| innodb_strict_mode                                | OFF                                                                                                                    |
| innodb_support_xa                                 | ON                                                                                                                     |
| innodb_sync_spin_loops                            | 30                                                                                                                     |
| innodb_table_locks                                | ON                                                                                                                     |
| innodb_thread_concurrency                         | 4                                                                                                                      |
| innodb_thread_sleep_delay                         | 10000                                                                                                                  |
| innodb_use_native_aio                             | ON                                                                                                                     |
| innodb_use_sys_malloc                             | ON                                                                                                                     |
| innodb_version                                    | 1.1.8                                                                                                                  |
| innodb_write_io_threads                           | 4                                                                                                                      |


A side note that might be relevant: I see that when I try to insert a large post (say over 10KB) from Drupal (which sits on a separate web server) to database, it lasts forever and the page does not return correctly. 

Regarding these, I'm wondering what should be my innodb_buffer_pool_size for optimal performance. I appreciate your suggestions to set this and other parameters optimally for this scenario. 

Your innodb_buffer_pool_size is enormous. You have it set at 20971520000. That's 19.5135 GB. If you only have 5GB of InnoDB data and indexes, then you should only have about 8GB. Even this may be too high.

Here is what you should do. First run this query

SELECT CEILING(Total_InnoDB_Bytes*1.6/POWER(1024,3)) RIBPS FROM
(SELECT SUM(data_length+index_length) Total_InnoDB_Bytes
FROM information_schema.tables WHERE engine='InnoDB') A;


This will give you the RIBPS, Recommended InnoDB Buffer Pool Size, based on all InnoDB Data and Indexes, with an additional 60%.

For Example

mysql>     SELECT CEILING(Total_InnoDB_Bytes*1.6/POWER(1024,3)) RIBPS FROM
    ->     (SELECT SUM(data_length+index_length) Total_InnoDB_Bytes
    ->     FROM information_schema.tables WHERE engine='InnoDB') A;
+-------+
| RIBPS |
+-------+
|     8 |
+-------+
1 row in set (4.31 sec)

mysql>


With this output, you would set the following in /etc/my.cnf

[mysqld]
innodb_buffer_pool_size=8G


Next, service mysql restart

After the restart, run MySQL for a week or two. Then, run this query:

SELECT (PagesData*PageSize)/POWER(1024,3) DataGB FROM
(SELECT variable_value PagesData
FROM information_schema.global_status
WHERE variable_name='Innodb_buffer_pool_pages_data') A,
(SELECT variable_value PageSize
FROM information_schema.global_status
WHERE variable_name='Innodb_page_size') B;


This will give you how many actual GB of memory is in use by InnoDB 
Data in the InnoDB Buffer Pool at this moment.

I have written about this before : What to set innodb_buffer_pool and why..?

You could just run this DataGB query right now rather than reconfiguring, restarting and waiting a week.

This value DataGB more closely resembles how big the InnoDB Buffer Pool should be + (percentage specified in innodb_change_buffer_max_size). I am sure this will be far less than the 20000M you have reserved right now. The savings in RAM can be used for tuning other things like


join_buffer_size
sort_buffer_size
read_buffer_size
read_rnd_buffer_size
max_connection


CAVEAT #1

This is very important to note: At times, InnoDB may require an additional 10% over the value for the innodb_buffer_pool_size. Here is what the MySQL Documentation says on this:


  The larger you set this value, the less disk I/O is needed to access
  data in tables. On a dedicated database server, you may set this to up
  to 80% of the machine physical memory size. Be prepared to scale back
  this value if these other issues occur:
  
  Competition for physical memory might cause paging in the operating
  system.
  
  InnoDB reserves additional memory for buffers and control structures,
  so that the total allocated space is approximately 10% greater than
  the specified size.
  
  The address space must be contiguous, which can be an issue on Windows
  systems with DLLs that load at specific addresses.
  
  The time to initialize the buffer pool is roughly proportional to its
  size. On large installations, this initialization time may be
  significant. For example, on a modern Linux x86_64 server,
  initialization of a 10GB buffer pool takes approximately 6 seconds.
  See Section 8.9.1, “The InnoDB Buffer Pool”.


CAVEAT #2

I See the following values in your my.cnf

| innodb_io_capacity                                | 200 |
| innodb_read_io_threads                            | 4   |
| innodb_thread_concurrency                         | 4   |
| innodb_write_io_threads                           | 4   |


These number will impede InnoDB from accessing multiple cores

Please set the following:

[mysqld]
innodb_io_capacity = 2000
innodb_read_io_threads = 64
innodb_thread_concurrency = 0
innodb_write_io_threads = 64


I have written about this before in the DBA StackExchange


May 26, 2011: About single threaded versus multithreaded databases performance
Sep 12, 2011: Possible to make MySQL use more than one core?
Sep 20, 2011: Multi cores and MySQL Performance


I just answered a question like this in Server Fault using a more concise formula:

SELECT CONCAT(CEILING(RIBPS/POWER(1024,pw)),SUBSTR(' KMGT',pw+1,1))
Recommended_InnoDB_Buffer_Pool_Size FROM
(
    SELECT RIBPS,FLOOR(LOG(RIBPS)/LOG(1024)) pw
    FROM
    (
        SELECT SUM(data_length+index_length)*1.1*growth RIBPS
        FROM information_schema.tables AAA,
        (SELECT 1.25 growth) BBB
        WHERE ENGINE='InnoDB'
    ) AA
) A;

