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Ah yes, Bit Torrent. The idea is simple; every download client is also
an upload server. And by sharing the load in this way, each client has
hundreds or even thousands of possible servers to choose from, and can
select the one with the best transfer rate from their location.
There's a big long paper about how all this works, but we can summarise
as follows:
- Every downloader is also an uploader.
- Each downloader connects to multiple uploaders.
- Downloaders try to download from the fastest uploader [from their
location on the network].
- Uploaders upload fastest to the peers they download fastest from.
Thus, hypothetically, the faster you upload, the faster you can
download. This encourages fairness.
- In the interests of fairness, once you've fully downloaded a file,
you're supposed to leave the torrent running for a while so that others
can download it from you.
Tools like Azureus even keep a track of your "share rating" for each
file, and a total rating. The better your share rating, the more you're
helping to seed the torrents you've downloaded.
Now, here's where it all falls apart:
- I have ADSL. That stands for ASYMMETRIC Digital Subscriber Line. In
laymen's terms, by download rate is about 5x faster than my upload rate.
I can download at the speedy rate of around 310 KB/sec, but I can only
*upload* at a maximum of about 46 KB/sec.
- This means that it is physically impossible for me to upload
particularly fast. That should hypothetically result in low download speeds.
- As it happens, seeders by definition don't download anything. So they
can't upload to others based on how fast they download form them. They
therefore upload to the fastest clients. So I still get a nice download
rate.
- However, this means that by the time I've downloaded a file, by share
rating for that file is about 1%.
- No problem, I can just leave the torrent running for a while...
- But wait! This torrent has 450 seeders and 3 downloaders. And I
sometimes see 200 KB/sec from a single seeder, so... why THE HELL is any
peer going to download from me at a piffling 40 KB/sec when there are
peers out there at least 5x faster?
- In short, I typically get upload rates of about 2 KB/sec (while
downloading or seeding). [No, I'm not firewalled.]
- The whole share rating thing is a bit short-signed too. If I upload
10MB to a 100MB torrent, I get a 10% rating. If I upload 100MB to a
1,000MB torrent... I get a 10% rating. In one case I've helped way, way
more, but I still get the same rating.
- More grave: If I upload 100MB to a 500MB torrent with 100 seeders and
3 downloaders, I get a 20% rating. If I upload 100MB to a 500MB torrent
with 3 seeders and 100 downloaders, I have helped out way, way more. But
I still only get a 20% rating.
In short, my ability to give is vastly smaller than my ability to take.
Not because I'm greedy, but because of the underlying technology. And BT
software rates me on how much data I upload, rather than on how helpful
I'm actually being. So Azureus is doomed to sit there for weeks
uploading 2 KB/sec to torrents that probably don't need the help anyway
just so my global share rating can approach unity...
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Orchid XP v8 wrote:
> And BT software rates me on how much data I upload,
You're aware that the share ratio isn't actually part of the protocol,
right? Nobody is keeping track of that except you and perhaps some private
trackers out there?
--
Darren New, San Diego CA, USA (PST)
Forget "focus follows mouse." When do
I get "focus follows gaze"?
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Orchid XP v8 wrote:
> - I have ADSL. That stands for ASYMMETRIC Digital Subscriber Line. In
> laymen's terms, by download rate is about 5x faster than my upload rate.
> I can download at the speedy rate of around 310 KB/sec, but I can only
> *upload* at a maximum of about 46 KB/sec.
>
> - This means that it is physically impossible for me to upload
> particularly fast. That should hypothetically result in low download
> speeds.
>
> - As it happens, seeders by definition don't download anything. So they
> can't upload to others based on how fast they download form them. They
> therefore upload to the fastest clients. So I still get a nice download
> rate.
>
> - But wait! This torrent has 450 seeders and 3 downloaders. And I
> sometimes see 200 KB/sec from a single seeder, so... why THE HELL is any
> peer going to download from me at a piffling 40 KB/sec when there are
> peers out there at least 5x faster?
The wikipedia article mentions some aspects of BT that specifically take
this into account.
> - The whole share rating thing is a bit short-signed too. If I upload
> 10MB to a 100MB torrent, I get a 10% rating. If I upload 100MB to a
> 1,000MB torrent... I get a 10% rating. In one case I've helped way, way
> more, but I still get the same rating.
But in all technicality, you've helped exactly the same, relative to the
whole torrent. Which is the only relevant measure.
--
Tim Cook
http://empyrean.freesitespace.net
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BTW, it was also designed for things like game patches, where you have
gazillions of people all downloading at once, with only a trickle of
interest after the initial flood of downloads.
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Darren New wrote:
> You're aware that the share ratio isn't actually part of the protocol,
> right?
Sure. I just feel like a bit of an arsehole downloading all this stuff
and then not giving anything back, that's all. :-}
...OK, I am officially weird.
[But also, the mathematics of the system is non-optimal. I hate
non-optimal!]
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Orchid XP v8 wrote:
> Darren New wrote:
>> You're aware that the share ratio isn't actually part of the protocol,
>> right?
>
> Sure. I just feel like a bit of an arsehole downloading all this stuff
> and then not giving anything back, that's all. :-}
I have a 900MB torrent up to share ratio 12.
Why not? I have no monthly bandwidth caps.
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On Sun, 17 Jan 2010 18:18:53 +0000, Orchid XP v8 wrote:
> Sure. I just feel like a bit of an arsehole downloading all this stuff
> and then not giving anything back, that's all. :-}
Just leave it run; I usually let my torrents run until either there are 0
peers for an extended period of time or until I hit a share ratio of
1:1. Sometimes I'll let them go longer, but I give priority to those
with a share ratio below 1:1.
Jim
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Orchid XP v8 wrote:
> - I have ADSL. That stands for ASYMMETRIC Digital Subscriber Line. In
> laymen's terms, by download rate is about 5x faster than my upload rate.
> I can download at the speedy rate of around 310 KB/sec, but I can only
> *upload* at a maximum of about 46 KB/sec.
>
> - This means that it is physically impossible for me to upload
> particularly fast. That should hypothetically result in low download
> speeds.
The download speed isn't tied to the upload speed like that. They are
independent.
Besides, the system works well for asymmetric connections like yours.
If a typical ratio of upload to download speed is 10x, then with 10
seeds you can top out your download speed, despite each seeder's upload
speed being low.
> - But wait! This torrent has 450 seeders and 3 downloaders. And I
> sometimes see 200 KB/sec from a single seeder, so... why THE HELL is any
> peer going to download from me at a piffling 40 KB/sec when there are
> peers out there at least 5x faster?
The file is broken up into blocks, you know, rather than transferred
sequentially.
So, the client will download one block from you, while downloading other
blocks from other seeds. You'll still be helping.
...Chambers
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>> - But wait! This torrent has 450 seeders and 3 downloaders. And I
>> sometimes see 200 KB/sec from a single seeder, so... why THE HELL is
>> any peer going to download from me at a piffling 40 KB/sec when there
>> are peers out there at least 5x faster?
>
> The file is broken up into blocks, you know, rather than transferred
> sequentially.
>
> So, the client will download one block from you, while downloading other
> blocks from other seeds. You'll still be helping.
Yes, but the BT algorithm is to try to select the peers which give you
the fastest download rate - to reduce leaching, but mainly so you
download from whichever peers are topologically "nearest" to you.
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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Nicolas Alvarez wrote:
> I have a 900MB torrent up to share ratio 12.
>
> Why not?
Becuase it might take several human lifetimes to reach that ratio?
--
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*
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