Sendfile and memory cache

Wawrzek

Could you please confirm or denied that system call sendfile(), opposite to read()+write() operations, does not use memory cache at all.

What means that i.e. in an Apache serving many times the same static content turning EnableSendfile Off might bust the performance, because files would be read from memory rather then from disk.

woboyle

The sendfile() function uses kernel instead of user-space memory. That means that your application heap will not be increased appreciably. However, the OS will still cache the data as that is not a matter of the application calling sendfile() as it is a matter of how the OS caches data read from disc. So, yes, it will use system memory to cache the data.

Wawrzek

Thanks for great response. As each good response it lead to further question.

Do data cached by kernel have to fit into 1GB of kernel memory?

Is there different in handling this memory between 32bit and 64bit version of x86 arch?

I'm asking because on a busy 32-bit server (apache serving static content - from 2 or 3 week non-stop) I see:

~ > free
             total       used       free     shared    buffers     cached
Mem:      16633136    3370264   13262872          0     520812    2391332
-/+ buffers/cache:     458120   16175016
Swap:      1052248         88    1052160

My estimation is that if the system kept all served files in the cache the memory would filled in few hours. So there are some software limit of the memory usage.

Wawrzek

I got confirmation that the reason of above issue is memory allocation limit in 32-bit kernel (as expected).