From mboxrd@z Thu Jan 1 00:00:00 1970 From: sdrb@onet.eu Subject: Variable download speed Date: Tue, 23 Feb 2016 12:19:58 +0100 (CET) Message-ID: Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset=US-ASCII To: netdev@vger.kernel.org Return-path: Received: from smtpo75.poczta.onet.pl ([141.105.16.25]:41487 "EHLO smtpo75.poczta.onet.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750955AbcBWL26 (ORCPT ); Tue, 23 Feb 2016 06:28:58 -0500 Received: from nowy9.kat.adbgroup.pl (unknown [91.230.58.65]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) (Authenticated sender: sdrb@onet.eu) by smtp.poczta.onet.pl (Onet) with ESMTPSA id 3q8dFq2BvZzT789rD for ; Tue, 23 Feb 2016 12:19:58 +0100 (CET) Sender: netdev-owner@vger.kernel.org List-ID: Hi, I've got a problem with network on one of my embedded boards. I'm testing download speed of 256MB file from my PC to embedded board through 1Gbit ethernet link using ftp. The problem is that sometimes I achieve 25MB/s and sometimes it is only 14MB/s. There are also situations where the transfer speed starts at 14MB/s and after a few seconds achieves 25MB/s. I've caught the second case with tcpdump and I noticed that when the speed is 14MB/s - the tcp window size is 534368 bytes and when the speed achieved 25MB/s the tcp window size is 933888. My question is: what causes such dynamic change in the window size (while transferring data)? Is it some kernel parameter wrong set or something like this? Do I have any influence on such dynamic change in tcp window size? sdrb