memcached problems...

Igor Sysoev is at
Thu Sep 3 12:51:35 MSD 2009

On Wed, Sep 02, 2009 at 05:56:04PM -0400, JG wrote:

> oh @#! forget it... my fault... i set the http port to 81 and was calling port 80 all the time in firefox. thats some way to spent time, too

I predict that you will see more CPU load and probably more IO load.
The reasons:

1) When nginx gets request from memcached, then 3 copy operations are done:
   from memcached to kernel, from kernel to nginx, from nginx to kernel.
   When file is sent from VM cache: this is a zero copy operation: sendfile()
   manages network card to get data from memory using DMA.

2) When a file is not in memcached, then nginx passes the request to
   a backend. The backend reads the file and stores it in memcached
   with 3 copy operations. If your current IO load is big, this
   means that physical memory is not enough to store hot content.
   memcached will decrease more memory available for VM cache resulting
   in more IO load.

Igor Sysoev

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