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  1. MariaDB Server
  2. MDEV-8932

innodb buffer pool hit rate is less than zero

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    Description

      When I set my.cnf like this:

      innodb_buffer_pool_size = 102400M
      		 
      innodb_buffer_pool_dump_at_shutdown=1
      		 
      innodb_buffer_pool_dump_now=1
      		 
      innodb_buffer_pool_load_at_startup=1
      		 
      innodb_buffer_pool_load_now=1

      and use （1-Innodb_buffer_pool_reads/Innodb_buffer_pool_read_requests）*100% to calculate innodb buffer pool hit rate ,I got a number less than zero,like 

      +----------------------+
      		 
      | hit_rate             |
      		 
      +----------------------+
      		 
      | -0.04231484607035707 |
      		 
      +----------------------+

      ,and 

      MariaDB [(none)]> show global status like 'innodb_buffer_pool_read%';
      		 
      +---------------------------------------+----------+
      		 
      | Variable_name                         | Value    |
      		 
      +---------------------------------------+----------+
      		 
      | Innodb_buffer_pool_read_ahead         | 0        |
      		 
      | Innodb_buffer_pool_read_ahead_evicted | 0        |
      		 
      | Innodb_buffer_pool_read_ahead_rnd     | 0        |
      		 
      | Innodb_buffer_pool_read_requests      | 13973819 |
      		 
      | Innodb_buffer_pool_reads              | 13979732 |
      		 
      +---------------------------------------+----------+

      I found Innodb_buffer_pool_reads is bigger than Innodb_buffer_pool_read_requests!

      When the server is loading pages at the start time,I execute show engine innodb status ,and I also see a negative number like:

      Buffer pool hit rate 18446744073709549183 / 1000, young-making rate 0 / 1000 not 0 / 1000
      		 
      Buffer pool hit rate 18446744073708648991 / 1000, young-making rate 0 / 1000 not 0 / 1000
      		 
      Buffer pool hit rate 18446744073709549537 / 1000, young-making rate 0 / 1000 not 0 / 1000
      		 
      Buffer pool hit rate 18446744073709551510 / 1000, young-making rate 0 / 1000 not 0 / 1000

      Obviously it's a negative number in unsigned long int.

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          relates to

          Bug - A problem which impairs or prevents the functions of the product. MDEV-21212 buf_page_get_gen -> buf_pool->stat.n_page_gets++ is a cpu waste (0.5-1%)

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed

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            elenst Elena Stepanova added a comment -

            Assigning to jplindst for advice on why it could be happening. I can only think of disk problems, but I suppose it would have been detected.

            elenst Elena Stepanova added a comment - Assigning to jplindst for advice on why it could be happening. I can only think of disk problems, but I suppose it would have been detected.
            linzhoukai Zhoukai Lin added a comment -

            There are several servers have this problem. So it may not be disk problems.

            linzhoukai Zhoukai Lin added a comment - There are several servers have this problem. So it may not be disk problems.
            jplindst Jan Lindström (Inactive) added a comment -

            Innodb_buffer_pool_read_requests indicates the the number of logical read requests (read from memory) InnoDB has done.
            Innodb_buffer_pool_reads indicates the number of logical reads that InnoDB could not satisfy from the buffer pool, and had to read directly from the disk (physical reads). This means that read_requests and reads may be different. You should calculate the hit ratio with

            Innodb_buffer_pool_read_requests / (Innodb_buffer_pool_read_requests + Innodb_buffer_pool_reads) * 100

            There seems to be a bug on calculation when buffer pool is loaded.

            jplindst Jan Lindström (Inactive) added a comment - Innodb_buffer_pool_read_requests indicates the the number of logical read requests (read from memory) InnoDB has done. Innodb_buffer_pool_reads indicates the number of logical reads that InnoDB could not satisfy from the buffer pool, and had to read directly from the disk (physical reads). This means that read_requests and reads may be different. You should calculate the hit ratio with Innodb_buffer_pool_read_requests / (Innodb_buffer_pool_read_requests + Innodb_buffer_pool_reads) * 100 There seems to be a bug on calculation when buffer pool is loaded.
            jplindst Jan Lindström (Inactive) added a comment -

            commit 4834d822efe5a812d251f254387b3f80beb9380d
            Author: Jan Lindström <jan.lindstrom@mariadb.com>
            Date: Wed Oct 28 08:42:51 2015 +0200

            MDEV-8932: innodb buffer pool hit rate is less than zero

            In buffer pool read hit rate calculation can lead rate that is
            less than zero.

            jplindst Jan Lindström (Inactive) added a comment - commit 4834d822efe5a812d251f254387b3f80beb9380d Author: Jan Lindström <jan.lindstrom@mariadb.com> Date: Wed Oct 28 08:42:51 2015 +0200 MDEV-8932 : innodb buffer pool hit rate is less than zero In buffer pool read hit rate calculation can lead rate that is less than zero.

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              jplindst Jan Lindström (Inactive)
              linzhoukai Zhoukai Lin
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