High Rate Internet Data Transfer for eVLBI

High Rate Internet Data Transfer for eVLBI

Ralph Spencer, Richard Hughes-Jones and Simon Casey

The University of ManchesterAll Hands Sept 2005

Radio Astronomy• The study of celestial objects at <1 mm to >1m

wavelength.• Sensitivity for continuum sources• B=bandwidth, integration time.• High resolution achieved by interferometers.

Some radio emitting X-ray binary stars in our own galaxy:

B/1

GRS 1915+105 MERLIN

SS433MERLIN and European VLBI

Cygnus X-1VLBA

GRS 1915+105: 15 solar mass BH in an X-ray binary: MERLIN observations

receding

600 mas = 6000 A.U. at 10 kpc

Very Long Baseline Interferometry

How interferometry works

• Interferometer: signals from each telescope brought together coherently for correlation (multiplication)

• Resolution /D << that possible from a single telescope, e.g the pimples on a golf ball at 3000 miles at the highest resolution

• Output of the correlator is one component of the Fourier transform of the sky brightness

Dcos

X

D

Earth-Rotation Synthesis

.

The baseline between each pair of telescopes forms an ellipse as the Earth rotates

The Aperture Plane (u,v)

• Need ~ 12 hours for full synthesis, not necessarily collecting data for all that time.

• Telescope data correlated in pairs: N(N-1)/2 baselines

• NBTrade-off between B and t for sensitivity ~sqrt(Bt)

• Data are calibrated, Fourier inverted and then deconvolved to give the images.

EVN

MERLIN

The European VLBI NetworkEVN

• Detailed radio imaging uses antenna networks over 100s-1000s km

• At faintest levels, sky teems with galaxies being formed

• Currently use disk recording at 512Mb/s (MkV)

• real-time connection allows greater – response– reliability– sensitivity

– Need Internet eVLBI

WesterborkNetherlands

Dedicated

Gbit link

EVN-NREN

OnsalaSweden

Gbit link

Jodrell BankUK

DwingelooDWDM link

CambridgeUK

MERLIN

MedicinaItaly

Chalmers University

of Technolo

gy, Gothenbu

rg

TorunPoland

Gbit link

eVLBI Milestones

• January 2004: Disk buffered eVLBI session:– Three telescopes at 128Mb/s for first eVLBI

image– On – Wb fringes at 256Mb/s

• April 2004: Three-telescope, real-time eVLBI session.– Fringes at 64Mb/s– First real-time EVN image - 32Mb/s.

• September 2004: Four telescope real-time eVLBI– Fringes to Torun and Arecibo– First EVN, eVLBI Science session

• January 2005: First “dedicated light-path” eVLBI– Australia to JIVE via Canarie: data from Huygens

descent transferred at ~450Mb/s

• 20 December 20 2004• connection of JBO to Manchester by 2 x 1 GE• eVLBI tests between Poland Sweden UK and Netherlands at 256 Mb/s

• February 2005• TCP and UDP memory – memory tests at rates up to

450 Mb/s (TCP) and 650 Mb/s (UDP)• Tests showed inconsistencies between Red Hat

kernels, rates of 128 Mb/s only obtained on 10 Feb• Haystack (US) – Onsala (Sweden) runs at 256 Mb/s

• Regular tests with VLBI data every ~6 weeks • 128 Mpbs OK, 256 Mpbs often,• But not 512 Mbps – WHY NOT?

• Correlator can cope with large error rates• up to 2 %• but need high throughput for sensitivity• implications for protocols

Gnt5-DwMk5 11Nov03-1472 bytes

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Throughput vs packet spacing Manchester: 2.0G Hz Xeon Dwingeloo: 1.2 GHz PIII Near wire rate, 950 Mbps UDPmon

Packet loss

CPU Kernel Load sender

CPU Kernel Load receiver 4th Year project

Adam Mathews Steve O’Toole

Gnt5-DwMk5 11Nov03/DwMk5-Gnt5 13Nov03-1472bytes

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Gnt5-DwMk5 11Nov03 1472 bytes

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Gnt5-DwMk5 11Nov03 1472 bytes

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UDP Throughput Oct-Nov 2003 Manchester-Dwingeloo Production

Throughput (JbGig1 - Mark608 @ JIVE via production)

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January 2005 repeated UDP over 2 days

ESLEA

• Packet loss will cause low throughput in TCP/IP

• Congestion will result in routers drooping packets: use Switched Light Paths!

• Tests with MB-NG network Jan-• JBO connected to JIVE via UKLight

in June (thanks to John Graham, UKERNA)

• Comparison tests between UKLight connections JBO-JIVE and production (SJ4-Geant)

UKLight Switched light path

Tests on the UKLight switched light-path Manchester : Dwingeloo

• Throughput as a function of inter-packet spacing

• Packet loss • Maximum size packets can

reach full line rates with no loss, and there was no re-ordering (plot not shown).

gig03-jiveg1_UKL_25Jun05

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gig03-jiveg1_UKL_25Jun05

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1472 bytes

Tests on the production network Manchester : Dwingeloo.

• Throughput

• Small (0.2%) packet loss was seen

• Re-ordering of packets was significant

gig6-jivegig1_31May05

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1472 bytes

UKLight using MkV terminals

The GÉANT2 Launch June 2005

Jodrell BankUK

DwingelooDWDM link

MedicinaItaly Torun

Poland

e-VLBI at the GÉANT2 Launch Jun 2005

UDP Performance: 3 Flows on GÉANT• Throughput: 5 Hour run 1500 byte MTU

• Jodrell: JIVE2.0 GHz dual Xeon – 2.4 GHz dual Xeon670-840 Mbit/s

• Medicina (Bologna): JIVE 800 MHz PIII – Mk5 (623) 1.2 GHz PIII 330 Mbit/s limited by sending PC

• Torun: JIVE 2.4 GHz dual Xeon – Mk5 (575) 1.2 GHz PIII

245-325 Mbit/s limited by security policing

(>600Mbit/s 20 Mbit/s) ?

• Throughput: 50 min period• Period is ~17 min

BW 14Jun05

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JodrellMedicinaTorun

BW 14Jun05

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JodrellMedicinaTorun

18 Hour Flows on UKLightJodrell – JIVE, 26 June 2005

• Throughput:• Jodrell: JIVE

2.4 GHz dual Xeon – 2.4 GHz dual Xeon

960-980 Mbit/s

• Traffic through SURFnet

• Packet Loss– Only 3 groups with 10-150 lost

packets each– No packets lost the rest of the

time

• Packet re-ordering– None

man03-jivegig1_26Jun05

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man03-jivegig1_26Jun05

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man03-jivegig1_26Jun05

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Packet

Loss

w10

Conclusion and Future directions• eVLBI needs to get to rates of 512 Mpbs – not there yet.

• Rates of 256 Mpbs on production networks possible now using TCP/IP in good conditions, but not higher rates.

• G2 launch showed continuous long term flows as in VLBI have different characteristics to those in short term tests (Iperf, UDPmon) : more investigation needed – collaborating with DANTE and SURFnet

• Packet loss is a killer for TCP – new protocols needed (ESLEA) - VSI-E standard

• The End Host Problem– The performance of Motherboards, NICs, RAID controllers and

Disks matter: Mk V units (Conduant)– MkV-B 2 Gbps data recording in 2 yrs?

• Next generation correlator 10 Gbps in 5-10 yrs

Thanks to

• Paul Burgess JBO• Steve Parsley, Arpad Szomoru, Cormac Reynolds JIVE• Nicola Pezzi, John Graham, Colin Greenwood, Peter

Clarke ESLEA• EVN observatories staff