Hi Godfrey, thank you very much for your very helpful input! See more
comments inline below.
Em 22-07-2013 13:55, Godfrey Chan escreveu:
Hi Rodrigo,
For the future, I suspect this question is more appropriate on
Stackoverflow/Quora than rubyonrails-core, but here is my quick take
on this:
On Monday, July 22, 2013 7:40:56 AM UTC-7, Rodrigo Rosenfeld Rosas wrote:
I'm tuning the performance of some request in my application and
there's
something puzzling me.
Rails reports the action takes 6ms:
Started GET "/templates/2.json" for 127.0.0.1 at 2013-07-22
10:49:15 -0300
Processing by TemplatesController#index as JSON
Parameters: {"id"=>"2"}
Rendered text template (0.1ms)
Completed 200 OK in 6ms (Views: 0.8ms | Sequel: 2.2ms)
Chrome's "waiting" time for a local request (localhost) is 25ms.
Then nginx will report 98ms for the same request when proxying the
Rails
app, but that is a separate issue I'll investigate by other means...
First, I'd like to understand what is this 6ms reported time by
Rails.
Does it take into account the whole request time since it reaches the
web server or only since it reaches the Rails stack? Does it take
into
account all middlewares? Any hint where those extra 19ms from
Chrome's
reported "waiting" time are being spent on?
The response time reported by Rails is based on a "process_action"
event in ActionController
<https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/instrumentation.rb#L18>,
so it's pretty low on the stack on only includes time spent in your
controllers. There is a Rack::Runtime middleware (included by Rails by
default) that wraps around your entire middleware stack (well, almost)
if you prefer a more accurate measurement. The middleware appends a
X-Runtime response header, which you can probably see from the Chrome
inspector panel. The difference between this value and the one in your
Rails log is probably pretty small though (a few ms) as a request
typically spends the majority of its time in the controller.
Thank you for pointing out to X-Runtime (I've seen it before but I was
unaware of its exact meaning). You're right, the reported Chrome's
timing when I'm accessing the Rails application directly (no nginx
proxy) is very close to the information in X-Runtime:
Chrome "Waiting" time: 27ms
X-Runtime: 0.025152
Reported time in the log: Completed 200 OK in 7ms (Views: 0.9ms |
Sequel: 3.0ms)
So, now I know that the overhead for ActionController::Base is about
18ms and I could try building some actions on top of Metal if I want to
improve it further, but at least now I know where to look at.
Nginx/apache/passenger etc might add 1ms or so to this but in the
grand scheme of things that's pretty much just noise.
You're right, when comparing the X-Runtime with Chrome's "Waiting" time,
the nginx overhead seems to be pretty small (4ms at most), but still
Rails will take longer to process the request when nginx is involved and
I have no clue why is that. Here's what happens when nginx is involved:
Chrome "Waiting" time: 88ms
X-Runtime: 0.083885
Completed 200 OK in 7ms (Views: 0.8ms | Sequel: 2.9ms)
This is what I'd really like to improve since there's no reason why
Rails should take longer to respond to the request when it is requested
by nginx instead of directly from the browser...
I'm using Puma for development mode (where I'm issuing those tests) and
unicorn under production...
I'd appreciate if you could provide me any hints on why the proxied
request is slower from the middleware perspective. Maybe some settings I
could try on nginx. Of course, I won't be insisting on that on this
mailing list since this is non-related to Rails, but in case you know
the answer I'd be glad to hear your guesses.
The "waiting" time in Chrome is roughly equivalent to the delta of
"responseStart" and "responseEnd" in the navigation timing spec
<https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html#processing-model>,
which means the time between immediately before browser sent the
request to your sever and when the browser receives the first byte of
the response header. This would include the time it takes to send the
request across the wire (watch your cookie size because they are sent
on every request!) and the time your server takes to generate a response.
The moral of the story is that if you care about the real world
performance of your app, you should probably looking into some kind of
RUM (real user monitoring)
<http://en.wikipedia.org/wiki/Real_user_monitoring> solution in
additional to your sever-side monitoring to get an accurate picture.
If you already using Newrelic you should enable it here
<https://newrelic.com/docs/features/real-user-monitoring>. There are
also other standalone services for doing this. (Shameless plug – if
you're building single page apps, we are working on this at caliper.io
<http://caliper.io>)
I wasn't aware of Caliper before. We're indeed maintaining an SPA and
I'll try to find some time to have a closer look at what Caliper is and
how it could help us. Currently we're indeed monitoring our application
with NewRelic and have RUM enabled for them (we have a set of
applications running actually, Grails, Rails, Solr and a Spring MVC
one). But I already have some hints of which requests I should be
optimizing (almost all JSON requests exclusively) and I'm also
converting some requests handling from the Grails app to the Rails one
and I'm particularly interested in making sure the requests are as fast
or faster when implemented in the Rails side (which has been true so
far, as the ported actions are much faster in the Rails side).
Also, I'd like to take the chance to ask about another thing I've
noticed recently. If I remember correctly, Rails used to fill in the
Etag automatically in every request based on the response content but
now it will only do that if you use something like fresh_when or
stale?.
Is that correct or has it always been working the way it currently
does?
I understand that the overhead may not be desired, but I'd be
curious if
the automatic Etag header has been removed, what would be the
reason for
that besides the overhead?
The "automatic" etag is generated by the Rack::ETag middleware and I
don't remember seeing this removed from Rails 4. To be sure, you can
always run rake middleware and check if you see something like "use
Rack::ETag" in the list.
Humm... Maybe I've seen then in the past for HTML requests and have just
assumed it worked for JSON format as well? I've confirmed the middleware
is enabled for my application. Maybe Rails will skip JSON requests since
jQuery won't take advantage of Not Modified responses by default. The
ifModified ajax parameter defaults to false and if you enable it you'll
have to take care of caching it in the client-side yourself, so maybe
that was enough reason for disabling the automatic ETag caching for JSON
requests... I've been handling this in jQuery with an ajax filter:
$.ajaxSetup({ cache: false, traditional: true, ifModified: true }); //
traditional is required by Grails default binding rules
var _ajaxCache = {};
$.ajaxPrefilter('json', function(options, originalOptions, jqXHR) {
options.success = function(data, textStatus, xhr) {
if (xhr.status != 304) {
if (xhr.getResponseHeader('Last-Modified'))
_mhAjaxCache[originalOptions.url] = data;
return originalOptions.success.apply(this, arguments);
}
var args = Array.prototype.slice.call(arguments);
args[0] = _ajaxCache[originalOptions.url];
return originalOptions.success.apply(this, args);
}
});
Thank you so much, Godfrey!
Cheers,
Rodrigo.
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