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Date: Thu, 18 Oct 2007 10:11:04 -0700 From: Rick Jones <rick.jones2@...com> To: Matthew Faulkner <matthew.faulkner@...il.com> Cc: netdev@...r.kernel.org Subject: Re: Throughput Bug? Matthew Faulkner wrote: > Hey all > > I'm using netperf to perform TCP throughput tests via the localhost > interface. This is being done on a SMP machine. I'm forcing the > netperf server and client to run on the same core. However, for any > packet sizes below 523 the throughput is much lower compared to the > throughput when the packet sizes are greater than 524. > > Recv Send Send Utilization Service Demand > Socket Socket Message Elapsed Send Recv Send Recv > Size Size Size Time Throughput local remote local remote > bytes bytes bytes secs. MBytes /s % S % S us/KB us/KB > 65536 65536 523 30.01 81.49 50.00 50.00 11.984 11.984 > 65536 65536 524 30.01 460.61 49.99 49.99 2.120 2.120 > > The chances are i'm being stupid and there is an obvious reason for > this, but when i put the server and client on different cores i don't > see this effect. > > Any help explaining this will be greatly appreciated. One minor nit, but perhaps one that may help in the diagnosis - unless you set -D (lack of the full test banner, or a copy of the command line precludes knowing), and perhaps even then, all the -m option _really_ does for a TCP_STREAM test is set the size of the buffer passed to the transport on each send() call. It is then entirely up to TCP as to how that gets merged/sliced/diced into TCP segments. I forget what the MTU is of loopback, but you can get netperf to report the MSS for the connection by setting verbosity to 2 or more with the global -v option. A packet trace might be interesting. Seems that is possible under Linux with tcpdump. If it were not possible, another netperf-level thing I might do is configure with --enable-histogram and recompile netperf (netserver does not need to be recompiled, although it doesn't take much longer once netperf is recompiled) and use the -v 2 again. That will give you a histogram of the time spent in the send() call, which might be interesting if it ever blocks. > Machine details: > > Linux 2.6.22-2-amd64 #1 SMP Thu Aug 30 23:43:59 UTC 2007 x86_64 GNU/Linux FWIW, with an "earlier" kernel I am not sure I can name since I'm not sure it is shipping (sorry, it was just what was on my system at the moment) don't see that _big_ difference between 523 and 524 regardless of TCP_NODELAY: [root@...pc105 netperf2_trunk]# netperf -T 0 -c -C -- -m 524 TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost.localdomain (127.0.0.1) port 0 AF_INET : cpu bind Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB 87380 87380 524 10.00 2264.18 25.00 25.00 3.618 3.618 [root@...pc105 netperf2_trunk]# netperf -T 0 -c -C -- -m 523 TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost.localdomain (127.0.0.1) port 0 AF_INET : cpu bind Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB 87380 87380 523 10.00 3356.05 25.01 25.01 2.442 2.442 [root@...pc105 netperf2_trunk]# netperf -T 0 -c -C -- -m 523 -D TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost.localdomain (127.0.0.1) port 0 AF_INET : nodelay : cpu bind Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB 87380 87380 523 10.00 398.87 25.00 25.00 20.539 20.537 [root@...pc105 netperf2_trunk]# netperf -T 0 -c -C -- -m 524 -D TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost.localdomain (127.0.0.1) port 0 AF_INET : nodelay : cpu bind Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB 87380 87380 524 10.00 439.33 25.00 25.00 18.646 18.644 Although, if I do constrain the socket buffers to 64KB I _do_ see the behaviour on the older kernel as well: [root@...pc105 netperf2_trunk]# netperf -T 0 -c -C -- -m 523 -s 64K -S 64K TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost.localdomain (127.0.0.1) port 0 AF_INET : cpu bind Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB 131072 131072 523 10.00 406.61 25.00 25.00 20.146 20.145 [root@...pc105 netperf2_trunk]# netperf -T 0 -c -C -- -m 524 -s 64K -S 64K TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost.localdomain (127.0.0.1) port 0 AF_INET : cpu bind Recv Send Send Utilization Service Demand Socket Socket Message Elapsed Send Recv Send Recv Size Size Size Time Throughput local remote local remote bytes bytes bytes secs. 10^6bits/s % S % S us/KB us/KB 131072 131072 524 10.00 2017.12 25.02 25.03 4.065 4.066 (yes, this is a four-core system, hence 25% CPU util reported by netperf). > sched_affinity is used by netperf internally to set the core affinity. > > I tried this on 2.6.18 and i got the same problem! I can say that the kernel I tried was based on 2.6.18... So, due dilligence and no good deed going unpunished suggests that Matthew and I are now in a race to take some tcpdump traces :) rick jones > - > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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