Why does uploading reduce download speed?

[cpeterso]

I have a cable modem that (according to speed tests) gets about 700 KBps down and 150 KBps up.

When I'm downloading a popular torrent with no Transmission Rate Limits, I often get about 50 KBps down and 150 KBps up. But if manually set my Upload Rate Limit to 30 KBps, my download speed often jumps to 150-200 KBps. Surprisingly, there seems to be an inverse correlation between upload speed and download speed. 30-50 KBps seems to be the sweet spot. Below an Upload Rate Limit of 30 KBps, downloads slow down.

If I'm not exceeding my cable modem's upload speed, why would decreasing my upload speed have such a big effect on my download speed? Is is my computer or router getting overworked/livelocked by too much uploading? I would assume that more uploading would give my client more "download credits" with seeders.

btw, I'm using Transmission 1.22+ (6275) on Mac OS X 10.4.

thanks,
chris

[theCrank]

that has nothing to do with transmission or bittorrent.

you need a tiny bit of your upstream to send an acknowledgment for each received data packet.

[texasranger]

What you are referring to is called "capping" where you limit, or "cap", your upload rate below your internet limit so the connection doesn't overload. If you have cable, this might seem to be a problem. With cable, bandwidth is shared within several houses, so if every one of those houses is using lots of bandwidth, the internet connection for each house gets smaller as it divides. You want to be sure to limit your upload rate to something less than your lowest upload rate possible. This tends to be hard with cable.

my guess is that you're not getting a CONSTANT 150 KB/s upload speed, which is probably why the internet is being overloaded. You're just going to have to deal with capping it at 30-50 KB/s like I do and upload when you have nothing to download.

[riddlemethis]

Riddle me this, then. Why is it that I see this effect even if the upload speeds are low?

If I cap uploads at, say, 30 KB/s (I can upload a constant 50 KB/s very easily on torrents and have) and the uploads are near 30 KB/s, they'll hit a constant value of 30.0 - and my download rate will climb.

If I'm uploading less than 50 KB/s between several torrents, with a global cap of 50 KB/s for the whole of them, but no individual caps, the download rates stay low. I have trouble seeing how this could be wholly due to bandwidth, when it seems like this is some sort of system-gaming feature. Is this possibly a feature of, say, torrent clients that they try to allocate upload bandwidth in a fashion that rewards/gains them bandwidth that benefits caps?

Because, y'know, I know it would be nice for me to upload 50 KB/s of a torrent instead of 20 or 30, but if the price is the torrent taking two weeks instead of 4-5 days, I'm not going to go that far to help my fellow torrenters while I get the file.

[cpeterso]

Instead of me manually tweaking my Upload Rate Limit, perhaps Transmission could auto-throttle itself (like TCP)?

For example, programmatically adjust the Actual Upload Rate Limit +10 or -10 KBps every 5 seconds, watching if the Total Download Speed increases or decreases.

[drcpebbles@yahoo.com]

I am using the same operating system. I limited the upload speed to 30 and saw a huge increase in download speed between 50-200kbs. Below 30 there is a dramatic drop as stated. Thanks a bunch for the hint and of course I will not limit the upload speed when I am not downloading! Not sure why it works this way though.

[Etabeeta]

When you download something from someone you first send something to them to let you know that you want to download, then you download. Usually you send a lot of these, so if you're upload is stuck on uploading other files, you can't send these notifications to others (upload). And without notifications you can't download.

It's something like that.

[essiw]

When you download something from someone you first send something to them to let you know that you want to download, then you download. Usually you send a lot of these, so if you're upload is stuck on uploading other files, you can't send these notifications to others (upload). And without notifications you can't download.

It's something like that.

to send those things are only a few bytes (not more then 1KB)
It is more that you reached your upload limit and that blocks your download speed (I can actual upload more then those websites say but I can't do anything else on the internet at that time)

[Debu]

I also have noticed this behavior so when I have slow download speeds I often play with the upload limit. It IS weird and I'm sorta glad someone else has noticed it. I'm on DSL.

When I do that I usually start off at 33kB/s - which is 1/2 of my available upload speed for torrents. Once the download flow gets going, I noticed that I can sometimes slowly increase the upload capacity without disturbing the flow.

[theelysium]

I don't think it has anything to do with the service. I myself have cable internet and I'm not bothered by any speed issues for torrents, except when I use other devices at the same time. I want to be able to use my Xbox online at the same time. Transmission seems to hog up a lot of upload, so I cap the upload, but I notice that when I do it lowers my download.

For example as we speak. I am uploading 124 - 218KBs and I am downloading 40 - 53KBs. If I cap my upload at 30KBs it will drop my download to about 8 - 19KBs. My speeds are of course fluctuating. If I turn the cap off of my upload the download speeds jump right back up to where they were before. I don't believe this has anything to do with my service, but the way the app was written. I think the app developers are trying to encourage you to share, which is the point of torrents right?

What I think is ratty though is that if I am getting an average of like 30KBs down and I make it match my upload I don't think I should be punished with a 8KBs download. I'm not getting it any faster the what I am caping the upload at, so why do that? It the only thing I don't like about Transmission.

They need to take that out of the program. Why give us a capping options if it's just going to hinder our download?

[phillipsjk]

For example as we speak. I am uploading 124 - 218KBs and I am downloading 40 - 53KBs. If I cap my upload at 30KBs it will drop my download to about 8 - 19KBs. My speeds are of course fluctuating. If I turn the cap off of my upload the download speeds jump right back up to where they were before. I don't believe this has anything to do with my service, but the way the app was written. I think the app developers are trying to encourage you to share, which is the point of torrents right?

I don't use Bittorrent that often, so have not run into this problem. I usually download at least twice as fast as I upload with no problems. On thing that may be happening is that you may be reducing the number of peers you are talking to. If for example (I don't know) Transmission tries to guarantee each peer 2kiB/s of bandwidth, dropping to 30kB/s from 150kb/s will drop you from 75 to 15 peers. You say your download rate dropped from ~50kB/s (667B/s per peer, assuming 75) to ~ 15kB/s (1kB/s per peer, assuming 15).

The Wikipedia article on the Bittorrent protocol has a cool graphic explaining how it works: http://en.wikipedia.org/wiki/BitTorrent_(protocol)

Edit: So far I have always downloaded from behind a NAT'ing router (I signed up to the forum because of problems poking holes in my router). I don't know if poking holes in the router would affect this.
Edit2: Because I don't mind waiting all day to download a large file, I also cap my download speed. Most recently at 1/3 of the bandwidth (100kB/s, 3Mbit connection). I was also downloading from a healthy swarm with 2:1 seeders:leechers

[zenzinin]

I know it's been 2 years since the first post, but I just started using Transmission for my Mac and I encounter this problem, which is how I got here via Google. The solution is as follows:

-If you are having very lowww download rates from very popular torrents (as in 100-200KBs download speed, which is incredibly low for me compared to other bittorrent clients I used when I was using PC), then most likely it has NOTHING to do with your UPLOAD rates. What you need to do is check for 1. is it being blocked/limited by your Mac or antivirus firewall (if it is, google how to allow access, but most likely if you are using Mac you probably don't even use any firewalls), and 2. most importantly you need to do PORT FORWARDing on your router. It doesn't matter wifi or for cable. If you don't know what port forwarding is, it's ok. Here is a website that tells you how to "optimize" the speed of your transmission client: http://www.techsupportalert.com/content ... -speed.htm which includes showing you step-by-step how to "port forward".

If the above things are checked and done, and you don't really mess with any other settings in Transmission client (meaning you left it to all default settings - if you've messed with it, don't fret, it should still be fine), then you should see a major increase in speed. I went from an average 150KBs to 400KBs after I did port forwarding. Of course, don't expect that kind of speed if you are downloading from a low seed (that is, relatively unpopular torrent).

-As for capping the UPLOAD rate, you should at least have it at 40... As for my reason, it's because it's nice to other users to share . Lol. And because it's "healthier" for your DOWNLOAD rates. Look at previous posts to understand why, but I wouldn't waste my time... I just read posts to get things working, not to understand why things work.