[PATCH] Cache: provide naive feedback loop for uncacheable URLs

[Maxim Dounin]

Hello!

On Mon, Apr 27, 2015 at 04:38:47PM -0700, Piotr Sikora wrote:

> # HG changeset patch
> # User Piotr Sikora <piotr at cloudflare.com>
> # Date 1430177825 25200
> #      Mon Apr 27 16:37:05 2015 -0700
> # Node ID 1e7878138228e3decb868f38eddc38937d55f403
> # Parent  96e22e4f1b03ff15a774c6ed34d74b897af32c55
> Cache: provide naive feedback loop for uncacheable URLs.
> 
> The cache lock provides a mechanism that allows only one request
> for the same URL to be sent upstream at the same time. All other
> requests must wait until the lock is released and/or not needed.
> 
> While this works as designed and protects upstream from requests
> stampede in case the response is cacheable, it results in a huge
> performance degradation for popular and uncacheable URLs, thanks
> to the artificial delay added to the upstream requests.
> 
> To make things even worse, the lock acquisition is retried every
> 500ms and prone to lock starvation - there is no queue, so newly
> accepted requests can jump ahead and acquire the lock before any
> of the already pending requests have a chance to retry it, which
> can lead to huge spikes in the response times and even timeouts.
> 
> This feedback loop provides a mechanism that allows requests for
> uncacheable URLs to immediately skip the cache lock.

I'm not really sure this is a right change.  In particular, because 
"uncacheable" is a property of a response, not URLs.

-- 
Maxim Dounin
http://nginx.org/