pavel cimbál, sven ubik cesnet tnc2005, poznan, 9 june 2005 tools for tcp performance debugging
TRANSCRIPT
Pavel Cimbál, Sven Ubik CESNET
TNC2005, Poznan, 9 June 2005
Tools for TCP performance debugging
Monitoring
Looking for performance limitations in different placesrequires different monitoring approaches communication segment monitoring type
• available bandwidth in network active & passive
• faulty network component active
• transport protocol behaviour* passive & end-host
• PC hardware performance end-host
• application & OS overhead end-host
* subject of this talk
TCP monitoring
TCP performance debugging tools:
• tcpdump + tcptrace + xplot
requires root access, follow-up analysis
loads CPU, can miss packets
• bulk• AIMD patch
Bulk tool
bulk: achievable throughput measurement (iperf-like)
+ synchronous monitoring of TCP runtime variables
• Reads kernel variables after each write() or read() call
• Does not require any kernel patch or root access
• Real-time analysis
• Low CPU overhead
• Can work together with AIMD patch to get more
information and to configure AIMD
Using bulk tool
Server side:bulk
Client side:bulk –c <hostname>
Set any socket option at any level: -s <optionlevel>,<optionname>
[,<initialiser1>[,<initialiser2> …]]
Get any socket option at any level: -g <optionlevel>,<optionname>
[,<member1>[,<member2> …]]
Option names for SOL_TCP level:
"TCP_NODELAY","TCP_MAXSEG","TCP_CORK","TCP_KEEPIDLE","TCP_KEEPINTVL","TCP_KEEPCNT","TCP_SYNCNT","TCP_LINGER2","TCP_DEFER_ACCEPT","TCP_WINDOW_CLAMP","TCP_INFO","TCP_QUICKACK"
Option members for TCP_INFO option:
"state","ca_state","retransmits","probes","backoff","options","wscales","rto","ato","snd_mss","rcv_mss","unacked","sacked","lost","retrans","fackets","last_data_sent","last_ack_sent","last_data_recv","last_ack_recv","pmtu","rcv_ssthresh","rtt","rttvar","snd_ssthresh","snd_cwnd","advmss","reordering"
Selected options to query
cwnd, ssthresh monitoring
bulk –gTCP_INFO,snd_cwnd,snd_ssthresh
Checking window scaling
195.113.147.2.50622 > 130.59.35.14.3141: S 1245641034:1245641034> (0) win 5840 <mss 1460,sackOK,timestamp 898882455 0,nop,wscale 4>
130.59.35.14.3141 > 195.113.147.2.50622: S 3938568209:3938568209> (0) ack 1245641035 win 17896 <mss 8960,sackOK,timestamp 1759960799> 898882455,nop,wscale 7>
bulk –gTCP_INFO,wscales
71
= 0x47, client wscale: 4, server wscale: 7
Bulk selected switches
-r reverse transfer, server to client
-b<block_size> block size for write(), read(),
adjusts result granularity!!
-c<connections> number of connectionsin sequence and/or in parallel
-m parallel mode
Added CPU load
% time seconds usecs/call calls syscall------ ----------- ----------- --------- -------------- 81.53 4.148857 61 67595 write 8.45 0.429789 6 66916 getsockopt 4.97 0.253101 4 66918 gettimeofday 0.00 0.000253 253 1 poll
strace –c bulk –gTCP_INFO,. . .
sending data
monitoring (+some user space load)
with bigger block size (-b) monitoring is less frequent
• Configuration of AIMD(x,y) parameters
cwnd=cwnd+x*MSS per ACK
cwnd=cwnd*y per packet loss
• Congestion Window Validation (CWV) and
Congestion Window Reduction (CWR) on / off / monitoring
• Works on a per-socket or per-system basis
• Available for Linux 2.4 and 2.6
AIMD patch
Example of per-system configuration:
sysctl –w net/ipv4/tcp_aimd 100 50 1 1
100% additive increase, 50% multiplicative decrease,CWV on, CWR on
AIMD patch, cont.
Example of per-socket configuration:
struct tcp_aimd aimd;
aimd.slope=100;aimd.ratio=50;aimd.cwven=1;aimd.cvwren=1;
setsockopt(sockfd, SOCK_TCP, TCP_AIMD, &aimd, sizeof(aimd));
AIMD patch, cont.
AIMD tuning
cesnet.cz → switch.ch, RTT=40 ms, win=4MB
aggressiveness vs. responsiveness vs. fairness
Download bulk + AIMD patch at:
www.ces.net/project/qosip#bulk
Future directions:
• Add hardware end-host monitoring (using PAPI and other
tools)
• Downloadable PC monitoring agent
Download and future work
Backup slides
Per-socket monitoring:
struct tcp_counters counters;
wsize=sizeof(counters);
getsockopt(sockfd, SOCK_TCP, TCP_COUNTER, &counters, &wsize);
AIMD patch, cont.