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#1 2021-06-02 12:51:57

JonnyRobbie
Member
Registered: 2015-04-28
Posts: 164

What is the purpose of a simultaneous downloads?

That title may have come across as bit too snarky, but it is a genuine question. My confusion stems from the fact that if you enable (for example) five simultaneous downloads, it should download each one five times slower, doesn't it? I mean, if you cap out your downlink with a single download, more in parallel won't help you with speed.
The only way I can imagine this to be useful is if your mirrors somehow to be extremely slow, so you'd download from multiple mirrors at the same time until you cap yourself out. But that really feels far fetched.
Did I miss anything else?

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#2 2021-06-02 13:22:53

dorian_greyscale
Member
Registered: 2021-06-02
Posts: 1

Re: What is the purpose of a simultaneous downloads?

I don't have a link, but the use case I saw cited awhile ago was that it's useful for when you have one large package that's holding up the entire process.

For example, it would likely reduce the time necessary to update if I can continue downloading other packages that are much smaller while downloading a Firefox update.

Yes you do need to split your download speeds, but it would be beneficial in order to prevent one slow/large package from bottlenecking the entire download process.

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#3 2021-06-02 13:23:37

Lone_Wolf
Member
From: Netherlands, Europe
Registered: 2005-10-04
Posts: 9,206

Re: What is the purpose of a simultaneous downloads?

JonnyRobbie wrote:

I mean, if you cap out your downlink with a single download, more in parallel won't help you with speed.

Correct, but that is often not the situation .


In 2017 network improvements in my neighbourhood lead to an increase in downloadspeed from 20 Mbps to 100 Mbps .
The mirror I used could 'only' handle 40 Mbps .
For  a while I used powerpill to make updating faster through parallell downloads.
My fav mirror was upgraded to a faster connection and I switched back to it.
DL speed varied from 60 to 90 Mbps depending on time of day .

In 2020 I switched providers and now have 210 Mbps download .
I found one mirror that could provde 130 Mbps occasionally, but it's typical speed was 70 Mbps .

with pacman 6 and parallel downloads I configured the best 5 (for me) and now get updates with 200 Mbps easily.


Disliking systemd intensely, but not satisfied with alternatives so focusing on taming systemd.
Did you use the guided installer ? If yes, I can't help you.

(A works at time B)  && (time C > time B ) ≠  (A works at time C)

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#4 2021-06-02 13:27:40

icar
Member
Registered: 2020-07-31
Posts: 167

Re: What is the purpose of a simultaneous downloads?

I have a similar situation. Usually I get a maximum of 30MB/s for each mirror I use. My home internet bandwidth is 600Mbps, soon 1Gbps. I get a big decrease in downloading time thanks to parallel downloads.

Last edited by icar (2021-06-02 13:28:09)

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#5 2021-06-02 13:35:35

JonnyRobbie
Member
Registered: 2015-04-28
Posts: 164

Re: What is the purpose of a simultaneous downloads?

I just tested my connection with speedtest and I got around 150 Mbps, which gives me around 18 MBps, which is about on par what I'm getting with my configured mirror and a single download.

So is each parallel download being serviced by a different mirror? Because I don't see how that would help even of you cap out your mirror's uplink.

Last edited by JonnyRobbie (2021-06-02 13:38:33)

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#6 2021-06-02 16:21:38

sabroad
Member
Registered: 2015-05-24
Posts: 181

Re: What is the purpose of a simultaneous downloads?

JonnyRobbie wrote:

if you enable (for example) five simultaneous downloads, it should download each one five times slower, doesn't it?

Default TCP slow-start means that it takes some seconds for a single TCP connection to saturate bandwidth. Where there are lots of small downloads, each over a series of separate TCP connections, it may never saturate available bandwidth.

Last edited by sabroad (2021-06-02 16:48:04)


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