High number connections-writing stuck

Valentin V. Bartenev vbart at nginx.com
Sun Dec 20 13:04:57 UTC 2015

On Friday 18 December 2015 15:55:47 Marcelo MD wrote:
> Hi,
> Recently we added a 'thread_pool' directive to our main configuration. A
> few hours later we saw a huge increase in the connections_writing stat as
> reported by stub_status module. This number reached +- 3800 and is stuck
> there since. The server in question is operating normally, but this is very
> strange.
> Any hints on what this could be?
> Some info:
> - Here is a graph of the stats reported, for a server with thread_pool and
> another without: http://imgur.com/a/lF2EL
> - I don`t have older data anymore, but the jump from <100 to +- 3800
> connections_writing happened in two sharp jumps. The first one following a
> reload;
> - The machines' hardware and software are identical except for the
> thread_pool directive in their nginx.conf. They live in two different data
> centers;
> - Both machines are performing normally. Nothing unusual in CPU or RAM
> usage. Nginx performance is about the same.
> - Reloading Nginx with 'nginx -s reload' does nothing. Restarting the
> process brings connections_writing down.

As I understand from your message everything works well.  You should also
check the error_log, if it doesn't have anything suspicious then there is 
nothing to worry about.

The increased number of writing connections can be explained by increased
concurrency.  Now nginx processing cycle doesn't block on disk and can
accept more connections at the same time.  All the connections that were
waiting in listen socket before are waiting now in thread pool.

  wbr, Valentin V. Bartenev

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