On 05/13/2012 05:20 PM, AlanO wrote:
I'm working with the HDS streaming technologies, within the Adobe
Flash world, and I'm trying to compile a set if 'FAQs' about each main
browser available to consumers and how it will react to the 'chunks'
being delivered via the HTTP stack.
I can answer some of your questions, though I'm not sure how they
specifically relate to HDS streaming.
1/ What is the default setting in Firefox for the Cache? Is it RAM
then Disk. Is this flow described anywhere?
We currently only use the RAM cache for for requests that can't be
stored on disk--no-store and certain SSL requests. So the flow as data
comes in is that data is delivered immediately to the program logic that
requested it, plus asynchronously scheduled to be written to the disk
cache.
For media requests (such as HDS), our media processing layer also has
its own, RAM-backed cache. I'm not sure what state the data is in
(decoded in any form yet) while it's in the media cache. But our media
cache is probably not getting used for Flash, so it's probably not
relevant for you. Our DASH implementation will use it, though.
Is there a specific issue related to caching and HDS that you're
concerned about?
2/ How is garbage collection instigated - the latest Firefox has an
option suggesting that, by default, there is an automatic garbage
collection which must use some rules... perhaps about disk space? For
example: in Google Chrome, the default cache can take up huge amounts
of disk space growing without some sort of size limit it seems, plus
you can only change the cache settings on the CLI of Chrome (no
'consumer' level configuration in the 'preferences' dialogs).
I assume by "garbage collection" you're referring to evicting items from
the HTTP cache (we usually reserve "garbage collection" for other things
not related to the cache, like collecting reference counted objects in
memory). Both Chrome and Firefox use a LRU (least-recently-used)
algorithm to evict items from the disk cache, and we also both use an
algorithm that grabs up to a certain % of disk drive space (the % drops
as the disk size increases), up to 1 GB max. In Firefox you can
easily provide a user-defined cache size instead--the UI for it is in
Preferences | Advanced | Network | Override automatic cache
management. Or you can set it in about:config by setting
'browser.cache.disk.smart_size.enabled' to false, and
'browser.cache.disk.capacity' to the size you want.
3/ What are the methods employed when multiple chunks are requested -
as with loading a web page, there is some 'pipeline' going on in the
network stack. Perhaps limiting the total number of working threads
for 'same domain' requests - this could impact on HDS chunks. Consider
where more than one HDS stream is present in a users browser (e.g.
multiple tabs/windows with an HDS stream) - if chunks are served from
the same FQDN, would there be a 'fight' due to slots within the worker
threads in the network stack?
Although I mention HDS here, this would likely go for any other
chunked streaming methods (MPEG-DASH, HLS) which may have a client
implemented inside a browser host.
We do have a bunch of limits for the maximum number of TCP connections
we allow to a server (or a proxy server, if that's being used). Search
for "connections" in about:config and you'll see them. Generally the
limit is 6 per FQDN (for persistent connections, which are the most
common) or 15 for non-keep-alive connections. When using a proxy we're
limited to 8 connections total (across all domains, IIRC), so that's a
more constrained case. These limits are also lower on mobile
Firefox--don't recall them off the top of my head, can let you know if
you need them.
We do have a queuing system for when there are more HTTP requests for a
domain pending than we have free connections (I'm assuming that's what
you mean by 'pipeline' here? It doesn't sound like HTTP pipelining per
se as in RFC 2616, which is a particular way of handling queued
connections, in some sense, but it sounds like you're talking about the
general case of "more requests than connections"). So if you have more
HDS streams (and/or other requests to the same domain) pending than the
above connection limits, some of them will have to wait for the others
to complete before being satisfied. It's definitely not a good idea to
request 8 media chunks in the reverse order that they'll be needed, for
instance.
Does that answer any of your questions? I think it might help to have
more of an idea of what problems/issues you're anticipating, or specific
info you need for your guide.
cheers,
Jason
Mozilla
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