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Re: Slow Windows uploads (with patch)

From: Daniel Jeliński <>
Date: Wed, 25 Jul 2018 15:15:09 +0200

2018-07-25 14:07 GMT+02:00 Christian Hägele <>:
> For me the SO_SNDBUF was 64KiB from beginning on and
> worked out well for my test-case. For me the send-ahead (number of
> unacknowledged bytes on the wire) was also fine.
> In both cases the throughput was similar in my quick tests, but I did not
> test it with high bandwidth and high latency, yet.

Thanks for your testing. It would be nice to know if 64KiB is the new
default in Windows 10, or if the initial buffer size is somehow
determined based on other connection properties. Please let me know
what you find.

> I still don't think that the SO_SNDBUF is the limiting factor here. Maybe
> that's because in my case the SO_SNDBUF is 64KiB by default and this is
> enough. I am really surprised that this change give you a significant
> improvement in performance. The only thing I can think of is that the
> 16KiB CURL_MAX_WRITE_SIZE and the 8KiB SO_SNDBUF don't work together very
> well.

In my case SO_SNDBUF was the limiting factor; I was able to get 200x
faster uploads just by increasing the buffer size to 4 MiB. But then,
different Windows versions may have different characteristics here.


Received on 2018-07-25