System is Slow on Windows? Look Here!

General
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I decided to post this in case it would help anyone who is also struggling out there using Syncthing with Windows machines.

TLDR - You may need to manually increase the UDP maximum READ/WRITE buffers for Windows because quic-go (used in Syncthing) might not automatically increase it. There’s a post on GitHub about increasing this for linux, but not for windows. You can find information about increasing the buffer for Windows here.

In my case, I have about 30 machines that must be Windows due to software limitations. Syncthing is keeping about 50GB of data synced between them from 1 folder, which works great 90% of the time. The other 10% is when an update occurs. Updates take all of the machines 2-3 days to recover with very small transfers occurring all the time.

After researching for a bit, I looked into the QUIC library that Syncthing was using, quic-go. In this library, there is a post about UDP buffers and how they impact the protocol.

Unfortunately, they didn’t mention how to fix this in Windows, but I eventually found 1 post and 1 document that specified that you can do it by adding two DWORD registry keys to HKLM:\SYSTEM\CurrentControlSet\Services\AFD\Parameters:

DefaultSendWindow - 0x17D7840
DefaultReceiveWindow - 0x17D7840

0x17D7840 = 25000000B = 25MB

Once I added the registry keys and restarted Syncthing…the significance was massive! A node was syncing 50GB of data in less than 10 minutes when previously it would take several hours to sync 5GB! Considering how my Syncthing “service” is set up, quic-go probably doesn’t have enough permissions to modify the buffer, so it is forced to work with the defaults, which is 8192…i.e. 8.1KB.

Hopefully, this helps someone, it helped me!

Note: Unfortunately, I had to delete some of my links because new users are limited to 2, but I kept the most important ones.

2 Likes
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Very nice! You should open an issue in the quic-go Github project. This is IMHO a valuable information which should be part of their wiki.

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Why aren’t you using tcp?

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I am using both. I had noticed that some nodes will swap from using TCP to using QUIC instead, which also happened to be the slow ones.

Admittedly, I haven’t looked into why it’s switching to QUIC in the first place when it could just use TCP. I think that should be the next step in my investigation.

That being said, I’ve also noticed performance increases in the nodes that are consistently using TCP.

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You can just force it to use tcp (by disabling quic listeners by adjusting the listen addresses), and that would also solve your problem, potentially in a better way

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Thanks AudriusButkevicius, I’ll test that out next. If it doesn’t maintain the TCP connection, there’s likely something in my network that I’ll need to analyze.

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tl;dr you’re basically enforcing huge send/receive buffers. The fact that this improves throughput is IMHO an indicator that you’re suffering from high packetloss. Your TCP connections getting dropped might also be a result of this.

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Thanks bt90! That’s a good follow-up post on quic-go’s Github. :sweat_smile: Haha, admittedly, I was a bit overzealous with increasing it to 25MB, and had a bit of a “go for broke” moment.

Initially, I had started with 2MB after reading the setup for the LCM Project: https://lcm-proj.github.io/lcm/content/multicast-setup.html#kernel-udp-receive-buffer-sizing

Unfortunately, I couldn’t include it in the original post since I’m limited on the links I can post.

The next step for me would be to decrease the buffers and figure out the ideal/sensible buffer size for my machines. I think I will need to do some research here. If I come up with something, I’d be happy to share my findings here in case it helps others.

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