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Before you get into trying to resolve issues with load there are
a few things to consider:
Your "model" of traffic is probably wrong...
- Have you seen this traffic shape.. if a user requests a page -
it will probably be a few milliseconds before the browser
requests the first static file, they will usually limit
themselves to something like 4, 6 or 8 parallel requests
(pipelining) to minimize the DNS/connect/handshake/disconnect
phases;
- Even if you have large numbers of simultaneous users the
amount of traffic won't be as bursty as you say - as they wont'
all hit "go" at the same time;
- The larger overheads are more likely to be up stream in
network etc.
Now to reduce load ...
- Look at a dedicated caching layer in front of apache. e.g.
varnish which can cache the static content; get your headers
right so that browsers + upstream caches cache your content;
- Look at the apache event mpm - which is much lighter than the
other mpms (prefork/threaded)
- Do you need 40 assets or can you do optimization on these
(e.g. merging css/js files) reducing images, icon fonts, css,
spriting etc; I have taken a site requiring 100s of assets and
gained by reducing these to 10-15...
- If you are worried about performance on such a small box then
redesign so that the site isn't heavy!
- Look at offloading some resources to 3rd party CDNs (e.g.
fonts core js-libraries etc)
Look at your hardware - if you are this worried - 1G is a very
small box - look at getting a larger server - most are virtual
anyway... then you need to look at the type of HDD etc..
- 1G is a small server you would get gains by having a bigger
server (and probably wouldn't cost much more!)
If you want this level of resiliance you probably need to look at
load balancing over multiple serves - then you can dedicate some to
static servers and some dynamic servers...
On 10/12/2016 14:22, Raphaël wrote:
Hi,
I've a question on how to prioritize traffic in order to optimize
the service in the case of traffic bursts:
Context:
* a server with finite resources (let's say 1 GB mem)
* a PHP application: initial page load needs 100 MB (index.php)
* for each page load (index.php) approx:
* ~ 40 subsequent assets (static files) are needed
* serving assets is, obviously, quicker than serving index.php
* I assume, and decide, that PHP-FPM must not use more than 700MB
* I want to avoid "broken" pages (missing assets/images/...) as much as possible
Thus PHP-FPM is configured to not allow no more than 7 children.
The Apache MaxRequestWorkers (worker MPM) is set to be strictly superior than
7*40 (lets say 350)
Now imagine a traffic burst with 200 distinct clients simultaneously
hitting the main page (wow!)
They now occupy 57% of the Apache workers, 193 of them waiting for a
PHP-FPM child. (<Proxy> "max" default value being ThreadsPerChild)
... some hundreds milliseconds later...
The 7 first clients having been served, each one now requests 40 more assets.
And the situation is then as follows:
* 7 hits on index.php were already processed successfully
* 7 currently being processed by PHP-FPM (still occupying Apache workers)
* 186 queued Apache workers hits /index.php, waiting for PHP-FPM/proxy-fcgi
* 7*40 = 280 new hits for assets (subsequent resources needed by the 7 first clients)
* 157 of them immediately get an available Apache worker and can be
served (157+186+7 == 350)
* >>>>>>> 123 assets will NOT get an available worker <<<<<<< PROBLEM HERE
In the "best" case these 123 requests, which should have been served
*now*, will end up in the ListenBackLog and wait the 157 first assets to
be served first and liberate their workers.
The server works virtually *as* if only 350-200 = 150 workers were
available (150 being < 280, which is the typical workers implication
for 7 pages-load)
200 being the (unpredictable/variable) "intensity" of the burst, I would
like to know of a better way to handle such a situation.
The first ideas that come to mind is service shaping (prioritization/quotas):
How to make Apache only accept 1/40 of the traffic to the fcgi php-fpm proxy.
Sample heuristic:
If all worker are used (350/350), we "compute" which proportion is
dedicated to index.php. If it's superior to a given configurable
threshold, then free some of the workers dedicated to this resources
in order to accept assets-directed resources.
I'm curious about possible solutions.
Thank you for reading.
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