question about proxy_buffers

[Maxim Dounin]

Hello!

On Thu, Jun 11, 2009 at 08:00:04PM +0200, Jérôme Loyet wrote:

> 2009/6/10 merlin corey <merlincorey at dc949.org>:
> > I'm not Igor so I cannot be totally sure, but my understanding of it
> > has always been 1- requests to the backend can use only one buffer and
> > the response gets buffered to disk.  As such, I generally start out
> > sites with log level info (might be too low) and try to tune down
> > requests getting buffered if at all possible.  I also try to stick
> > with magic numbers good for the architecture (so 8 and 16 based,
> > generally).
> >
> Thx for the answer merlin,
> 
> I did some test. The request I make to nginx is proxied to the
> backend. There is only one request made at the same time.
> I request a 5Mo file.
> 
> First: with minimum buffers:
> proxy_buffers 2 4k;
> 
> I have this in the log: 2009/06/11 19:40:05 [warn] 24607#0: *104 an
> upstream response is buffered to a temporary file
> /LIBRE/nginx/proxy_temp/0000000000 while reading upstream, ....
> 
> Then I set 10 buffers of 1Mo:
> proxy_buffers 10 1m;
> and nothing appears in logs.
> 
> This tells me that a request can use more than one buffer.

Yes.

> I'll set up small buffers but a large number:
> proxy_buffers 2048 8k;
> 
> Is it too much ?

Each connection that uses proxy can use up to 16M of memory with 
this setting (even after actual connection to backend has been 
already closed).  With 1024 client connections this means up to 
16G of memory, probably a bit too many.

And keep in mind that:

- Doing reconfiguration will spawn new worker processes and memory 
  usage by nginx nearly doubles for a while.

- You also need some memory for OS's disk cache.

Some hints about seveal big vs. many small buffers:

- Using bigger buffers may help a bit with CPU usage, but usually 
  it's not a big deal.

- Using bigger buffers means less disk seeks if reply won't fit 
  into memory and will be buffered to disk.

- On the other hand, small buffers allow better granularity (and 
  less memory wasted for last partially filled buffer).

Since there are different load patterns (and different hardware) 
it's hard to recommend some particular setting, but the about 
hints should allow you to tune it for your system appropriately.

Maxim Dounin


> 
> >
> > 2009/6/9 Jérôme Loyet <jerome at loyet.net>:
> >> Hi all,
> >>
> >> I'm not sure how proxy_buffers are used.
> >>
> >> 1- requests to the backend can use only one buffer. If the reponse is
> >> bigger than one buffer, some part of the response is buffered to disk.
> >> 2- requests to the backen can use multiple buffers. If the response is
> >> bigger than on buffer, nginx will use more buffers (if available of
> >> course).
> >>
> >> So depending on the answer, I would use :
> >> 1- big buffers so that 80% of the requests can feet in on buffer. And
> >> enough buffers so that 80% of the requests can be buffered
> >> 2- small buffers and a lots of buffers
> >>
> >> don't know what to use :) any clue ?
> >>
> >> thx
> >> ++ jerome
> >>
> >>
> >
> >
>