Comments on the fastestmirror plugin

[Bill Davidsen]

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.

> 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.

-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot