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Date: Wed, 24 Feb 2016 10:59:23 +0100 (CET) From: sdrb@...t.eu To: Rick Jones <rick.jones2@....com> cc: netdev@...r.kernel.org Subject: Re: Variable download speed On Tue, 23 Feb 2016, Rick Jones wrote: > On 02/23/2016 03:24 AM, sdrb@...t.eu wrote: >> 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? > > > If an application using TCP does not make an explicit setsockopt() call to > set the SO_SNDBUF and/or SO_RCVBUF size, then the socket buffer and TCP > window size will "autotune" based on what the stack believes to be the > correct thing to do. It will be bounded by the values in the tcp_rmem and > tcp_wmem sysctl settings: > > > net.ipv4.tcp_rmem = 4096 87380 6291456 > net.ipv4.tcp_wmem = 4096 16384 4194304 > > Those are min, initial, max, units of octets (bytes). > > If on the other hand an application makes an explicit setsockopt() call, > that will be the size of the socket buffer, though it will be "clipped" by > the values of: > > net.core.rmem_max = 4194304 > net.core.wmem_max = 4194304 > > Those sysctls will default to different values based on how much memory is in > the system. And I think in the case of those last two, I have tweaked them > myself away from their default values. > > You might also look at the CPU utilization of all the CPUs of your embedded > board, as well as the link-level statistics for your interface, and the > netstat statistics. You would be looking for saturation, and "excessive" > drop rates. I would also suggest testing network performance with something > other than FTP. While one can try to craft things so there is no storage I/O > of note, it would still be better to use a network-specific tool such as > netperf or iperf. Minimize the number of variables. > > happy benchmarking, Hi, To be honest I don't know if "wget" uses setsockopt() in this case. As you suggested I observed the system information while using wget. "top" shows that kernel thread which is used by ethernet driver uses 100% of first cpu and "wget" application uses at the same time 85% of second cpu. More interesting are the information from vmstat: procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 0 0 119688 4 81096 0 0 0 0 55 24 0 6 93 0 0 0 0 0 119664 4 81132 0 0 0 0 632 1223 0 0 100 0 0 0 0 0 119664 4 81156 0 0 0 0 640 1300 0 0 100 0 0 0 0 0 119664 4 81156 0 0 0 0 632 1284 0 0 100 0 0 0 0 0 119664 4 81152 0 0 0 0 633 1283 0 0 100 0 0 0 0 0 119604 4 81148 0 0 0 0 637 1292 0 0 100 0 0 0 0 0 119604 4 81148 0 0 0 0 634 1289 0 0 100 0 0 0 0 0 119604 4 81144 0 0 0 0 637 1395 0 0 100 0 0 0 0 0 119604 4 81140 0 0 0 0 639 1296 0 0 100 0 0 0 0 0 119604 4 81132 0 0 0 0 633 1282 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 638 1298 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 635 1288 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 634 1283 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 626 1273 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 634 1287 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 633 1286 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 635 1399 0 0 100 0 0 0 0 0 119604 4 81128 0 0 0 0 638 1287 0 0 100 0 0 0 0 0 119604 4 81136 0 0 0 0 633 1286 0 0 100 0 0 0 0 0 119468 4 81168 0 0 0 0 669 1422 1 0 99 0 0 start of wget at 14MB/s 4 0 0 119444 4 81240 0 0 0 0 3541 6869 0 18 82 0 0 4 0 0 119444 4 81276 0 0 0 0 10200 20032 4 55 40 0 0 4 0 0 119444 4 81276 0 0 0 0 10175 19981 3 57 39 0 0 4 0 0 119444 4 81272 0 0 0 0 10170 19986 5 57 37 0 0 5 0 0 119444 4 81272 0 0 0 0 10158 19950 4 60 36 0 0 6 0 0 119412 4 81272 0 0 0 0 9711 19316 7 56 37 0 0 3 0 0 119460 4 81288 0 0 0 0 1828 3400 4 89 7 0 0 speed 25MB/s 4 0 0 119460 4 81288 0 0 0 0 1674 3044 9 89 2 0 0 4 0 0 119460 4 81288 0 0 0 0 1606 2929 4 93 2 0 0 4 0 0 119460 4 81288 0 0 0 0 1560 2832 2 93 4 0 0 5 0 0 119460 4 81284 0 0 0 0 1552 2806 3 94 3 0 0 4 0 0 119460 4 81280 0 0 0 0 1624 2945 2 95 3 0 0 5 0 0 119460 4 81276 0 0 0 0 1228 2165 6 93 1 0 0 end of wget transmission 0 0 0 119580 4 81276 0 0 0 0 1066 1935 2 26 72 0 0 0 0 0 119604 4 81280 0 0 0 0 608 1043 0 0 100 0 0 Looks like when the wget transmission starts at 14MB/s - there are a lot of interrupts and context switchings (irqs: 10000 cs:20000), but when the speed increases to 25MB/s number of interrupts decreases to about 1500. I suppose that in such a case the ethernet driver is suspected. I've tested also as you suggested the throughput using iperf - and there is no such significant difference in download speed like in wget case. sdrb
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