Every so often people ask me the question how should they estimate memory consumption by MySQL Server in given configuration. What is the formula they could use.

　　经常有人问我配置MySQL时该如何估算内存的消耗。那么该使用什么公式来计算呢？

　　The reasons to worry about memory usage are quite understandable. If you configure MySQL Server so it uses too small amount of memory it will likey perform suboptimally. If you however configure it so it consumes too much memory it may be crashing , failing to execute queries or make operation to swap seriously slowing down. On now legacy 32bit platforms you could also run out of address space so that had to be watched as well.
Having said so, I do not think looking for the secret fomula to compute your possible memory usage is the right approach to this problem. The reasons are - this formula is very complex nowadays and what is even more important “theoretically possible” maximum it provides have nothing to do with real memory consumptions. In fact typical server with 8GB of memory will often run with maximum theoretical memory usage of 100GB or more. Furthermore there is no easy “overcommit factor” you can use - it really depends on application and configuration. Some applications will drive server to 10% of theoretical memory consumptions others only to 1%.

　　关心内存怎么使用的原因是可以理解的。如果配置MySQL服务器使用太少的内存会导致性能不是最优的；如果配置了太多的内存则会导致崩溃，无法执行查询或者导致交换操作严重变慢。在现在的32位平台下，仍有可能把所有的地址空间都用完了，因此需要监视着。
话虽如此，但我并不觉得找到什么可以计算内存使用的秘诀公式就能很好地解决这个问题。原因有 -- 如今这个公式已经很复杂了，更重要的是，通过它计算得到的值只是“理论可能”并不是真正消耗的值。事实上，有8GB内存的常规服务器经常能运行到最大的理论值 -- 100GB甚至更高。此外，你轻易不会使用到“超额因素” -- 它实际上依赖于应用以及配置。一些应用可能需要理论内存的 10% 而有些仅需 1%。

　　So what could you do instead ? First take a look at global buffers which are allocated at start and always where - these are key_buffer_size, innodb_buffer_pool_size, innodb_additional_memory_pool_size, innodb_log_buffer_size, query_cache_size. If you’re using MyISAM seriously you can also add the size of Operation System cache you would like MySQL to use for your table. Take this number add to it number of memory Operation System and other applications need, add might be 32MB more for MySQL Server code and various small static buffers. This is memory which you can consider used when you just start MySQL Server. The rest of memory is available for connections. For exampe with 8GB server you might have everything listed adding up to 6GB, so you have 2GB left for your threads.

　　那么，我们可以做什么呢？首先，来看看那些在启动时就需要分配并且总是存在的全局缓冲 -- key_buffer_size, innodb_buffer_pool_size, innodb_additional_memory_pool_size, innodb_log_buffer_size, query_cache_size。如果你大量地使用MyISAM表，那么你也可以增加操作系统的缓存空间使得MySQL也能用得着。把这些也都加到操作系统和应用程序所需的内存值之中，可能需要增加32MB甚至更多的内存给MySQL服务器代码以及各种不同的小静态缓冲。这些就是你需要考虑的在MySQL服务器启动时所需的内存。其他剩下的内存用于连接。例如有8GB内存的服务器，可能监听所有的服务就用了6GB的内存，剩下的2GB内存则留下来给线程使用。

　　Each thread connecting to MySQL server will needs its own buffers. About 256K is allocated at once even if thread is idle - they are used by default thread stack, net buffer etc. If transaction is started some more space can add up. Running small queries might only barely increase memory consumption for given thread, however if table will perform complex operations such as full table scans, sorts, or need temporary tables as much as read_buffer_size, sort_buffer_size, read_rnd_buffer_size, tmp_table_size of memory might be allocated. But they are only allocated upon the need and freed once given stage of query is done. Some of them are allocated as single chunk at once others, for example tmp_table_size is rather maximum amount of memory MySQL will allocate for this operation. Note it is more complicated than once may think - multiple buffers of the same type might be allocated for exampe to handle subqueries. For some special queries memory usage might be even larger - bulk inserts may allocate bulk_insert_buffer_size bytes of memory if done to MyISAM tables. myisam_sort_buffer_size used for ALTER TABLE, OPTIMIZE TABLE, REPAIR TABLE commands.