Cluster:LowLatency

Plan (updated August 28, 2005)

• Decide on two benchmarks, one micro (low-level) and one application, probably MD with GROMACS in a form that's latency bound on bazaar & Cairo. Set these up on bazaar and Cairo and start tests.

• Formal literature search and preliminary review.

• Test tcp_low_latency - does it work? If so, what does it do _exactly_

• Show that latency is in the network stack, either by reference or by test (preferrably the former since it will take less time).

Notes

• Comparisons & Instrumentation

• • Web100
    • http://www.psc.edu/~mathis/papers/mathis03web100.ps
    • http://www.web100.org/

• • Comparison of time between layers in network stack. Patch only for 2.6.9.
    • http://user.informatik.uni-goettingen.de/~kperf/
    • http://user.informatik.uni-goettingen.de/~kperf/gaug-ifi-tb-2005-03.pdf

• • Comparing while varying link delay times
    • http://developer.osdl.org/shemminger/LK2004_TCP.pdf
    • http://developer.osdl.org/shemminger/LWE2005_TCP.pdf
    • Reno - Linux 2.4
    • Westwood - Wireless
    • Vegas
    • Binary Increase Congestion Control (BIC) - Default for Linux 2.6

• • Lots of discussion
    • http://www.ggf.org/Public_Comment_Docs/Documents/May-2005/draft-ggf-dtrg-survey-1.pdf
    • HS-TCP
    • Scalable TCP
    • FAST TCP
    • BIC & CUBIC
    • Westwood
    • .....

• Misc
  • Dynamic Right Sizing (DRS)
  • Window Scaling RFC
    • http://www.faqs.org/rfcs/rfc1323.html
  • Modifying buffer sizes manually
    • http://proj.sunet.se/E2E/tcptune.html
  • Kernel options
    • http://www.lam-mpi.org/MailArchives/lam/2005/01/9529.php
  • Default Linux TCP/IP options field contains:
    • Maximum Segment Size(MSS) http://rfc.activedomain.org/0500-0999/rfc0879.html
    • Selective Acknowledgement(Sack) ftp://ftp.isi.edu/in-notes/rfc2018.txt
    • Timestamping http://www.faqs.org/rfcs/rfc1323.html
  • opensolaris code
    • http://cvs.opensolaris.org/source/xref/usr/src/uts/common/inet/

• Low Latency Kernel Option
  • net.ipv4.tcp_low_latency sysctl
    • /proc/sys/net/ipv4/tcp_low_latency
  • From /usr/src/linux/Documentation/networking/ip-sysctl.txt

       tcp_low_latency - BOOLEAN
       If set, the TCP stack makes decisions that prefer lower
       latency as opposed to higher throughput.  By default, this
       option is not set meaning that higher throughput is preferred.
       An example of an application where this default should be
       changed would be a Beowulf compute cluster.
       Default: 0

Ideas

• Reduce error checking to improve performance
• Reduce number of memory copies, maybe by not being fully TCP/IP-compliant?
• Look into STP (scheduled transfer protocol)

Links

• Linux Kernel Howto
• The Linux Kernel Online Book
• Conceptual Architecture of the Linux Kernel (paper)
• The Linux Headquarters
• Kernel Newbies