[Hackage] #447: do parallel builds

Duncan Coutts duncan.coutts at worc.ox.ac.uk
Sun Jan 18 09:18:35 EST 2009

On Sun, 2009-01-18 at 12:54 +0900, Curt Sampson wrote:
> On 2009-01-17 17:16 +0000 (Sat), Duncan Coutts wrote:
> > On Wed, 2009-01-14 at 00:09 +0900, Curt Sampson wrote:
> > 
> > > ..parallel downloads are a very happy thing; in some circumstances I can
> > > run a dozen downloads in parallel, without any individual one being
> > > slower than it would be running alone.
> > 
> > Presumably to different servers though right?
> No; to a single server. There still appear to be a lot of TCP
> implementations out there not supporting the extensions necessary to
> increase window size beyond 64K, meaning that at any time only 64K of
> unacknowledged data can be outstanding on the connection. For me, a
> 250 ms. round-trip-time (RTT) is not unusual, meaning that the minimum
> amount of time between the server sending a segment and getting my
> client's acknowledgement of receipt is 250 ms. That means that, no
> matter how much bandwidth is available, I'll never see more than 256
> KB/sec, or so, which is a small fraction (1/40th) of the available
> bandwidth between me and a server in the US with at least a 100 Mbps
> connection.
> > I was also under the impression that most web servers kind of frowned on
> > more than one or two connections from the same client and some would
> > take active measures to prevent it.
> Not that I'm aware of. Most have this capability, and some people chose
> to use it when they it solves a problem for them, but I haven't found it
> to be that terribly common.
> And of course there's also the case where the files one downloads are
> on different servers, either because the one "server" from which you're
> downloading is actually a cluster (a common case with larger sites) or
> because the things your downloading are actually hosted by different
> entities.

Fair enough.

I'm rather jealous. My ADSL is only 0.5Mbit/s :-(

So we'd need to establish multiple Network.HTTP.Browser sessions (though
with a way to control the maximum number of them for people with DSL
like mine). Another thing to bear in mind for the redesign of the
cabal-install download component.


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