←back to thread

Why HN was slow and how Rtm fixed it

(ycombinator.com)
257 points pg | 9 comments | 19 Jan 11 18:41 UTC | HN request time: 1.322s | source | bottom
Show context
rarrrrrr ◴[19 Jan 11 20:49 UTC] No.2121225[source]▶
Since no one has mentioned it yet - Varnish-cache.org, written by a FreeBSD kernel hacker, has a very nice feature, in that it will put all overlapping concurrent requests for the same cacheable resource "on hold", only fetch that resource once from the backend, then serve the same copy to all. Nearly all the expensive content on HN would be cacheable by varnish. Then you can get it down to pretty close to "1 backend request per content change" and stop worrying about how arbitrarily slow the actual backend server is, how many threads, how you deal with the socket, garbage collection, and all that.
replies(4): >>2121261 #>>2121274 #>>2121319 #>>2122946 #
1. j_baker ◴[19 Jan 11 21:05 UTC] No.2121274[source]▶
Can't you only use varnish for mostly non-dynamic content? Like for example, wouldn't the fact that it displays my username and karma score at the top of the page make it so that you couldn't use varnish (or at least make it more difficult)?
replies(2): >>2121326 #>>2121328 #
2. dminor ◴[19 Jan 11 21:23 UTC] No.2121326[source]▶
Yes, you'd have to somehow separate the dynamic content from the static content so they could be fetched in different requests and then combined (probably via ajax). If it's just your username and karma then it's simple enough, but if the comments are displayed differently for different people then it could be bear.
replies(2): >>2121485 #>>2121585 #
3. seiji ◴[19 Jan 11 21:23 UTC] No.2121328[source]▶
Check out http://www.varnish-cache.org/trac/wiki/ESIfeatures

There could be a private internal URL to just return username and karma to populate the user info header.

replies(1): >>2121733 #
4. yummyfajitas ◴[19 Jan 11 22:13 UTC] No.2121485[source]▶
Comments are displayed differently for different people.

I once posted a comment which was immediately invisible to everyone besides me - I'm guessing it was marked as spam for some reason, but left visible to me so I think I successfully posted it.

5. rarrrrrr ◴[19 Jan 11 22:44 UTC] No.2121585[source]▶
I think the only way PG would try this is if it's clear that javascript, client side hacks, etc are not required -- and I think that's the case.

Aside from the header, there's only a relatively small number of variations for any given content, right? Showdead, ability to downvote, etc? So, each of these variations gets a distinct ESI URL. Like /item_$showdead_$downvote_$etc right in the internal URL, so any combination of these is a distinct URL. Only the first user to hit any particular combination would result in a request to the backend, and that could remain in cache until its content changed. No wizardry required.

6. j_baker ◴[19 Jan 11 23:23 UTC] No.2121733[source]▶
Doesn't that kind of defeat the purpose though? The point of using varnish is that it keeps you from having to access the backend altogether. This is getting into an area where something like memcache might be more appropriate.
replies(1): >>2121765 #
7. danudey ◴[19 Jan 11 23:34 UTC] No.2121765{3}[source]▶
Well, the point of using varnish is to keep you from having to access the backend any more than is absolutely necessary. It's incredibly trivial to generate HTML showing a user's username and karma, and even if it weren't it could be stored in memcached. Generating the front page, the comments pages, etc. is the hard part, and varnish can keep that from being generated any more than is necessary.
replies(1): >>2121924 #
8. j_baker ◴[20 Jan 11 00:17 UTC] No.2121924{4}[source]▶
Of course, but I seem to recall pg writing at some point that one of the goals of HN being to prove that "slow" languages can scale using caching. I assume, therefore, that he already has caching of some kind in place for those things. If varnish isn't going to save an access to the server (which seems to be the primary thing that's slowing things down), what value is varnish providing above what pg already has in place?
replies(1): >>2121932 #
9. danudey ◴[20 Jan 11 00:21 UTC] No.2121932{5}[source]▶
The requests won't queue up as badly because the server will be able to clean out 'simple' requests in a much lower time than generating much larger pages. They won't queue up as much because the requests take less time to handle, so they can be cleared out faster than they come in (compared to larger requests that queue up faster than they can be handled).