question about proxy_buffers

[Jérôme Loyet]

Le 12 juin 2009 07:45, Jérôme Loyet<jerome at loyet.net> a écrit :
> 2009/6/12 Maxim Dounin <mdounin at mdounin.ru>:
>> Hello!
>>
>> On Fri, Jun 12, 2009 at 12:30:58AM +0200, Jérôme Loyet wrote:
>>
>> [...]
>>
>>> >> 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.
>>>
>>> Do you mean that proxy_buffers are dedicated to each connection ?
>>> I thought it was shared by all connections.
>>
>> Directive proxy_buffers specify maximum number and size of buffers
>> that can be used by one (each) connection.  Not every connection
>> uses so many buffers as they are allocated on demand, but every
>> connection can do so.
>>
>> Original documentation clearly states "... number and size of
>> buffers for one connection ..." (in russian):
>>
>> http://sysoev.ru/nginx/docs/http/ngx_http_proxy_module.html#proxy_buffers
>>
>> Probably this was lost in translation (or was clarified later).
>> Feel free to fix wiki.
>>
> OK Thanks you very much. In this case 16Mo (2048 * 8k is way way too much)
>
> I'll analyse my logs to know what is the distribution of my requests
> size so that I can do the better compromise. Thx you guys

After analysing my logs. I have found out:

1- on the dynamic part of the site : more than 80% of requests are
less or equal than 32ko
2- on the static part of the site : more than 80% of requests are less
or equal than 12ko

So I'll set this configuration.
proxy_buffers 4 8k; #32Ko to handle about 88% of static requests
proxy_buffers 4 4k; #16Ko to handle about 90% of dynamic requests

it seems more secure :) what do you think ?

>>
>>>
>>> >
>>> > 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
>>> >> >>
>>> >> >>
>>> >> >
>>> >> >
>>> >>
>>> >
>>> >
>>>
>>
>>
>