Cache Hit Latency for large responses

Context:
I am currently using nginx to serve as a Response Cache Proxy in order
to
shim-out (isolate) network latency while running some performance
related
tests.

The Perf Tests are comprised of 50 repetitions of the same set of
requests.
=> I have been able to successfully set-up the proxy_cache so that it
would
Cache the requests.
(I’m using the request_uri and request_body as the cache_key, since I
need
to Cache POST Requests as well)
(Our POST Requests are not necessarily Write Requests since our API is
using
the POST Params for cases where we need to specify longer Params that
exceed
the limit on GET)
=> I am logging the Cache HIT Status and Request_time in the access_log
and
basically, I am seeing a variance between 500 ms ~ 2 seconds for the
Same
Request being Served from the Cache even when it’s both a HIT. The Size
of
the Response is around 170K, and the request_body length is 65k. Most of
the
other HITS are like 0~100ms for request_time and only the ones that are
made
around that huge request seems to have some latency.

Question:
#1 I’m using proxy_cache_path. Is it possible that disk I/O is causing
this
latency? I thought the Filesystem Cache was able to handle these Cache
HITs
in memory.

#2 When I set the cache_key to be a combination of the request_body and
request_uri itself, when it does the lookup, would it automatically pull
those and to the lookup as expected?

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,249791,249791#msg-249791

Hello!

On Thu, May 01, 2014 at 06:27:19PM -0400, lovekmla wrote:

(Our POST Requests are not necessarily Write Requests since our API is using
#1 I’m using proxy_cache_path. Is it possible that disk I/O is causing this
latency? I thought the Filesystem Cache was able to handle these Cache HITs
in memory.

This depends on OS and filesystem you are using, as well as amount
of memory you have.

#2 When I set the cache_key to be a combination of the request_body and
request_uri itself, when it does the lookup, would it automatically pull
those and to the lookup as expected?

ENOPARSE, but note that $request_body variable only works if a
request body is small enough and fits in client_body_buffer_size.


Maxim D.
http://nginx.org/

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