Comments on the fastestmirror plugin
Patrick O'Callaghan
pocallaghan at gmail.com
Mon Mar 15 03:08:36 UTC 2010
On Sun, 2010-03-14 at 20:57 -0400, Bill Davidsen wrote:
> Patrick O'Callaghan wrote:
> > On Sat, 2010-03-13 at 16:52 -0500, Bill Davidsen wrote:
> >> Patrick O'Callaghan wrote:
> >>> The yum fastestmirror plugin (yum-plugin-fastestmirror) claims to
> >>> evaluate the speed of a bunch of repo mirrors and use the fastest one
> >>> relative to the user's location.
> >>>
> >>> However AFAIK what it *actually* does is make a test connection to the
> >>> to the candidate mirrors and order them according to response time,
> >>> which in many cases is dominated by network latency, which can distort
> >>> the results. For well-connected user machines in first-world countries
> >>> it probably doesn't matter much, and may have the beneficial effect of
> >>> spreading the load over a wider range of mirrors, but for those of us in
> >>> a less privileged position it can matter a lot. Ironically, these are
> >>> the cases where such an optimization could do the most good.
> >>>
> >> And there you have the heart of the problem, the evaluation is not remotely
> >> correct for most cases. It would be worth adding code to download some small RPM
> >> from a number of sites and measure b/w for something real. However, disabling
> >> the feature works, too.
> >
> > Sadly, downloading a "small" RPM is unlikely to give very reliable
> > results either. Due to TCP slow-start, a stable effective b/w may only
> > be reached after some 10's of kb have been downloaded.
> >
> Wow, I wasn't talking dialup. Tens of kb is a few ms on anything useful for
> major D/L, the slowest DSL I've ever seen sold was 768kb, or max of ~80kB/s.
> Figure 100kB per site and stop after the ten fastest are within 5-10% of each
> other (that's probably max line speed). Base the speed calc on the last 10k of
> 100kB to get the steady state value. You don't really care if the process is
> slow, once the table is built you use it. Actually, rechecking the five fastest
> is probably practical.
That's kind of what I was saying actually. Maybe we have different ideas
of what "small" means. My DSL line is 1Mbps and of course it's not just
used for updating Fedora :-)
Anyway, the general idea jibes with what I'm thinking. I'll be sure to
let people know if I actually do it, which is vanishingly unlikely but
not strictly impossible.
> > This is not an easy problem to solve.
> >
> Fortunately, you don't have to find the best, virtually all of the top sites
> will be close unless they're overloaded. The perfect is the enemy of the good
> enough.
That's certainly true (except when dealing with security, but that's a
horse of a different colour).
poc
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