Upload killing download

Amraam · 3 Feb 2016 at 22:25

Hi all,

I've just been trying to upload about 1.5GB of files to my google drive. Unfortunately, when uploading, it almost completely killed my download speed for the whole LAN. The upload speed is only 2Mb (about 1100kbps according to the SkyHub) but getting 20Mbps out of our "up to" 17Mbps.

Anyways, the LAN is only at 100Mbit until I finish setting everything up, but that shouldn't be the problem. As soon as I finished the upload the download went immediately back to normal. There are no QoS features I can see on the SkyHub, so is there anything else I can check to see why the download speeds plummet when I'm uploading?

I'm guessing it's just maxing out the upload bandwidth and acknowledgements aren't getting back properly?

Thanks

Felon · 3 Feb 2016 at 22:42

Amraam said:
I'm guessing it's just maxing out the upload bandwidth and acknowledgements aren't getting back properly?

Basically this.

When you upload data on a DSL connection with such a small amount of upload bandwidth, TCP will "window up" to the maximum speed it can and use the whole 2Mbps (or 1100Kbps in your case with is pretty tiny)

When you then start a download at the same time, the TCP transfer rate depends on how well it can window up, it can only window up if it can send lots of acks back to the peer, if the upload is congested these acks will be delayed and also incur latency, one of the biggest killers of TCP throughput is latency.

To make matters worse, QoS won't really make any difference either - even with an expensive router and all the QoS settings in the world, you're still limited by the tiny queues and buffers on the DSL interface, and if you have a mixture of big packets and small packets (data + acks) on the wire at the same time, the large data packets simply take too much time to process, and delay the acks, which means less windowing, and less throughput.

Steveocee · 4 Feb 2016 at 07:42

As per above.

If you're maxxing out your upload it's expected your download will be in bits. You need more bandwidth.

Amraam · 4 Feb 2016 at 08:22

Screeeech said:
Basically this.

When you upload data on a DSL connection with such a small amount of upload bandwidth, TCP will "window up" to the maximum speed it can and use the whole 2Mbps (or 1100Kbps in your case with is pretty tiny)

When you then start a download at the same time, the TCP transfer rate depends on how well it can window up, it can only window up if it can send lots of acks back to the peer, if the upload is congested these acks will be delayed and also incur latency, one of the biggest killers of TCP throughput is latency.

To make matters worse, QoS won't really make any difference either - even with an expensive router and all the QoS settings in the world, you're still limited by the tiny queues and buffers on the DSL interface, and if you have a mixture of big packets and small packets (data + acks) on the wire at the same time, the large data packets simply take too much time to process, and delay the acks, which means less windowing, and less throughput.

Steveocee said:
As per above.

If you're maxxing out your upload it's expected your download will be in bits. You need more bandwidth.

Thanks guys, makes sense. If I got to a 40/10 connection, will I not just incur the same issue, albeit with a 10Mbps upload?

wesley · 4 Feb 2016 at 11:31

best thing to do is limit the upload speed by 10% will improve the download speed massively

i learnt that when using bit torrent 10 years ago

Avalon · 4 Feb 2016 at 11:41

As above, most file sync/storage services have an upload speed limit setting for exactly this kind of situation, use it and you shouldn't have a problem.