Low Latency Networking
Glenford Mapp
Digital Technology Group
Computer Laboratory
http://www.cl.cam.ac.uk/Research/DTG/~gem11
What is Latency?
• The time taken to send a unit of data
between two points in a network
• A low latency network is a network in
which the design of the hardware, systems
and protocols are geared towards
minimizing the time taken to move units of
data between any two points on that
network
Throughput
• Number of bytes of data that is transferred
per second between two points
• Doesn’t high throughput imply low latency?
• Not necessarily
– A bus vs a car travelling along a section of road
• Which has the higher throughput?
• Which has the lower latency?
Throughput vs Latency
• In simplest form,
– Throughput ~ C / Latency
– C = instantaneous capacity
• Number of units that are handled per operation
• So if C is large you can get good throughput even
if your latency is not low
• Low latency does not necessarily imply high
throughput if C also gets smaller
– ATM is a good example
Throughput Claims
• Look carefully at high throughput claims.
– Have they decreased the latency
• Per unit operation is faster
– Software -> Hardware (ATM)
– Have they increased instantaneous capacity
• Serial -> Parallel-Parallel->Serial
• In most designs we have a mixture of both
– Manufacturers will generally allow increased
latency if capacity greatly increases
Who cares about latency?
• Why is latency important?
• Some applications are more affected by
latency rather than throughput
– Voice
• Also affected by jitter
– Networked Games
– Interactive sessions
Lessons from Computers
• Consider the Mainframe in the time-sharing
era. 1963-1976
• Studies showed that user productivity
reduced by half if the response time from
mainframe increases from 0.5 to 3 seconds
• Mainframe optimised for throughput
– Maximize the number of people using it
• High throughput
Lessons from Computers
• But as more people logged on the slower
the machine became and by noon the
response time would increase markedly so
user productivity would fall
• Key factor in the development of PCs
• Famous saying
– I love the Alto (first PC) because it does not run
faster at night!
A look at the Internet
• Not really designed for low latency
• Designed to be adaptable and robust
• But the new applications we want the
Internet to support need low latency
– Web servers
– Voice over IP
– Networked Games, etc
Components of Network Latency
• Hardware
– Different hardware capacities and limitations
• Ethernet – variable packet size; max 1500
• ATM – 53 bytes uses fixed cells
• Network Routers and Switches
– Queueing strategies
– Overload/ Congestion strategy
Components of Network Latency
• System Latency
– Moving the packet between the application and
the network interface
– OS latency
• The operating system handling the packet
– Application Latency
• Application must acquire resources (e.g. CPU) in
order to send or consume data
Traditional Networking –
A closer look
• Look at a packet being received by the host
machine and delivered up to the application
• At the lowest level, packet enters the
network interface card (NIC) – ends up in a
buffer or fifo on the card. Card generates an
interrupt.
Tradition Networking cont’d
• Interrupt Handler runs, data is moved into a
system buffer in main memory.
• Packet is placed on a receive queue
– In Linux there is one network receive queue
• Packets from all the network interfaces are placed
on that queue
• Packet is marked for system processing
– Interrupt Handler ends
Traditional Networking cont’d
• System processing
– Packet is taken up the protocol stack
• IP processing ; TCP processing
– Connection information associated with the
packet is used to find the corresponding socket
• Socket ~ Src (IPaddr, TCP port) , Dest (IPaddr, TCP port)
Traditional Networking cont’d
• Queue the packet on the socket structure
and see if any application threads are
waiting for incoming data
• If so, copy the data from system buffer to
the user buffer and wake up the thread
• Application has to wait until it gets the CPU
to consume data
Analysis of Traditional
Networking
• Interrupt systems – potentially infinite latency
– Processing of packets in the queue is affected by the
rate of incoming packets
• Copying data adds to latency
• OS sits between two worlds
– It de-multiplexes the packet and decides its final
destination
– It also ensures that the relevant application is scheduled
to receive the data. This is called application
synchronisation
APPLICATION
LAYER
Socket Interface
Socket layer in OS
System
Buffers
System
Buffers
NIC
Network
Cross Talk Issues
• Interrupt level
– while an application is running on the
processor, network interrupts occur on
incoming packets for other processes.
• Protocol level
– packets for all applications are multiplexed and
de-multiplexed in the kernel
• Application Level
– All applications must share resources so
sometimes I must wait a long time before I get
the processor.
Some ways to improve
Traditional Networking
• User level network interfaces
– UNET - Matt Walsh (1995-1998)
• Zero copy architectures
– Virtual memory mapping techniques
• Vertical Partitioning of Operating Systems
UNET
• Application has an interface to talk directly
to a network device
• Doesn’t involve the kernel in things like
protocol processing, etc.
• Uses per application message queues to
send and receive data
• Novel idea at the time
– complicates what applications need to do
UNET Endpoint
Recv Free
queue queue
Communication segment
Send
queue
Zero-Copy Architecture
• No need to copy data up to the application
• DMA from network buffers in NIC card
straight into system buffers
• Use VM techniques to map the relevant
system buffers into the address space of the
application
Vertical Partitioning of the OS
• So UNET gave applications an abstract
network card so there was less multiplexing
of data.
• Why not go all the way and do more
partitioning of OS resources
• So CPU is carefully partitioned, file systems
and disk devices also carefully partitioned
Pegasus project - Cambridge
• Studied system support for multimedia
applications
• Developed a new operating system called
Nemesis which adopted a vertical approach
– Most of the operating system functions were in
shared libraries which executed in the user’s
process space
– System-wide page table, so no copying
Vertical Approach
Processes
Shared Libraries
Normal
OS
Why haven’t these ideas been
universally implemented
• Some were explored
– VIA is a hardware idea based on UNET
– Replace PCI bus
– Devices have receive, send and completion
queues and are connected along a high-speed
serial bus
– One or two products out there but fell out of
favour
• Infiniband - now popular – extension of VIA
Ideas not universal
• Zero copy and VM ideas explored in some
Operating Systems, e.g. the Spring OS by
Sun. Some ideas made their way into
Solaris. Windows 2000 and XP, via Mach
and NT
• Nemesis was too radical for prime time
– QoS ideas have been taken up by others
But the real reason was..
• That processor and network speeds have
been increasing fast enough to keep
traditional networking in the picture.
• If you simply want to browse the Web and
read email, then it is OK
• However, there is a looming problem
Network speeds still going up!
• We have gone from 10 Mbps in 1987 to
10G in 2004 and beyond.
• Processor not be able to keep up
– Interrupt rate is phenomenal
• Buses like the PCI bus cannot keep up
– Move to PCI Express (Switch Fabric)
• Workstation can presently saturate the
network but the tide is rapidly turning!
• Network traffic will soon be able to cripple your PC
Need a system that is less
interrupt-dependent
• Two main approaches
– No OS processing whatsoever
• including no interrupts
• data is moved by hardware
• OS is used to setup where the data is moved to
– Apply more processing power but target it on
the network interface
Shared Memory Model
• Data transfer is accomplished by writing to
memory addresses in the local address
space of the process
• This data is captured by the local network
card and serialized into packets which are
transferred over the network to the remote
machine which writes the data to remote
addresses.
How does it actually work?
• A region of the local address space of the
process is mapped to an IO region on the
card. That mapping is usually made using
standard memory-mapping techniques.
– In Unix the mmap call is used.
• Same thing is done on the remote side
Shared Memory Model
Process VM
Process VM
NIC
NIC
packets
How is the association between
the local and remote regions
made
• Fixed
– In early SMMs, it was fixed.
– All processors on the network share the same
region.
• Flexible
– Needs a communications channel to set up the
mapping between regions
Fixed SMM
Process VM space
Proc A
Proc B
Proc C
Proc D
Dynamic SMM
Process VM space
Proc A
Proc B
Proc C
Proc D
SMM
• Been around a long time
– Used to communicate between processors in a
cluster.
• The SMM is divided into pages, some of
which can be mapped between two
processes and the other set can be mapped
globally
Problems with SMM
• Since no interrupts are involved and the OS
is no longer in the loop, it’s hard to inform
the remote node that data has been sent and
is waiting to be read
• Major problem is therefore not the transfer,
but application synchronization
Applications Synchronization
Solutions
• Polling:
– the receiver keeps polling certain addresses to
see if a data transfer has occurred
– This is expensive (wasting local CPU) and only
relevant if there is a real chance of a data
transfer.
– Could be used to provide to provide a form of
distributed synchronization - spinning on a
remote address
Application Synchronization
Solutions
• VM signalling
– Pagefault or access violations
– Example: page is only mapped locally when
there is data to be read. If I access the page
when there is no data, then a pagefault occurs
and I am blocked until the owner writes to the
page
VM Signalling
• If I wish to read and there is data to be read
then the page is mapped into my address
space read-only.
• If I attempt to write to the page, a pagefault
occurs and I am blocked until I can acquire
the write lock for the page
• Not scalable, too closely coupled to the VM
system
Out-of-Band signaling
• Use a separate channel outside the data
transfer region to signal that data has been
transferred.
• For example, writing to a special set of
addresses would cause an interrupt to be
generated at the remote end
Out-of-Band Signalling
• So you would transfer the data by writing to
your local address
• After you then wrote to a special address
associated with that memory region
• An interrupt occurs on the other side and
the OS works out which buffer you are
referring to and wakes up the waiting
process
Out-of-Band Signalling
• Out-of-Band Signalling still involves the
processor to achieve application
synchronization
• Adds the overall transfer latency
– Ex. Memory Channel
• data transfer 2.9 us
• acquire spin lock 120 us
• Increases the expense of the NIC
History of SMM
• Used to be extremely proprietary
• DEC Memory Channel best known
– Used a fixed shared memory region of 512 MB
divided into 64K pages each page being 8K
– Very versatile, can share pages between one or
more processes. Use broadcast facilities
– Average latencies 10-25 us
SCI - Scalable Coherent Interface
• IEEE Standard 1956-1992
• Uses high speed unidirectional links
– Parallel links 16 bits, 500 Mhz (8 Gbs)
– Serial G-Link technology (1Gbs)
• Packet-based transfer
– header - 16 bytes; data = 0, 16, 64 or 256 bytes
– queue and signal interrupts
SCI cont’d
• Can do cache-coherency (optional)
• Latency < 10 us
• Modern cards uses 64bit and 66 MHz buses
(5.33 Gbits/s)
• Big player: Dolphin Interconnect
– Sun uses their boards to build megaservers
Processor Intensive Approach
PIA
• We offload networking by using a processor
on the NIC
• Myrinet - most well-known exponent
– Full duplex data links 2 Gbits/s
– Bus 64-bit 133Hz PCI-X bus
– PC - 255 Mhz RISC & Memory
Myrinet con’t
• Packet-based
– Header, packet type, payload
• Host Computer controls the NIC
– runs a MCP program
• Myrinet controls around 39 % of the cluster
market
Performance
• Latency around 6.3 us
– Climbs to over 100 us over 10000 bytes
• One way throughput 248 MB/s
– Messages over a 1000 bytes
• Two way throughput 489 MB/s
– Message over 10000 bytes
• Throughput between Unix processes on
different hosts
– 1.98 Gbits (uni) 3.9 Gbits/s (bi)
Comparing SCI and Myrinet
• Latency are about the same
• SCI much faster for cluster of 8 or less
– but slows exponentially as the number of PCs
increases
• Myrinet is better for large systems > 64
• Software appears more complete with
Myrinet
Recent developments in Low
Latency Systems
• Collapsed LAN project (CLAN)
– 1997 - 2002, AT&T Laboratories-Cambridge
– project originally centred around using fibre
technology throughout the building
– remoting PCs; just have mouse, keyboard and
display in your office and put the PC in the
server room
– bought some SCI cards and got some systems
going
CLAN project
• Faced the application synchronization
problem
• Came up with a novel solution called
Tripwire
– in-band synchronization
– an event is signalled on the receiver when data
is written to a special address in the data region
during the data transfer
Tripwire
Processes
Tripwire
CLAN Project
• Applications can therefore set Tripwires and
be notified when they occur
– no spinning, no extra hardware for out-of-band
signaling
• Latency:
– DWORD - RRT = 3.7us
– 1KB IP transfer - 225 Mbit/s RRT= 100us
– Throughput 910 Mbits/s 33 MHz, 32 bit bus
Will Low latency ever make it
into the Main Stream
• Some low latency 1 Gigabit/s NICs on the
market
• Unfortunately 1 Gigabit/s market is now in
the commodity phase.
• Real battle is shaping up at 10 Gbit/s
market
– CLAN project -> Level5Networks-> Solarflare