Re: TCP speed

["Matt Wakeley" <matt_wakeley@agilent.com>]

Y P Cheng wrote:

> Julo wrote:
> >Our experience is the same. TCP is FAST.
> >The only remaining trouble is memory copy from TCP buffers
> >to application buffers. Unless
> >handled properly this may slow you down considerably.
>
> The issue is not the TCP memory to memory copy speed, it is the latency time
> of receiving TCP acknowledges.  Between two endpoints of New York and Los
> Angeles, latency is in milliseconds if not in seconds.  On a one-gigabit
> network, for each millisecond there are 100K of data, or 66 1.5K datagrams
> being transferred.  In fibre channel, there is this EE-credit, End-to-End.

This is used in Fibre Channel class 1 and class 2 service, neither of which is
mainstream fibre channel.  Most implementations use class 3 - no EE credit.

But I fail to see your point.  You state how FC has EE credit, and make it sound
like a good think, and then say it's a bad reason for using TCP...

>
> If the sending party has 10 EE credits, it can't send more than 10
> datagrams.  EE-credit manages the TCP sliding window currently discussed in
> iSCSI.  After sending 10 datagrams, one must wait for acknowledges that may
> take several hundred milliseconds to come.
>
> I do believe TCP is a wrong protocol for iSCSI.  A SCSI request from an
> initiator is inherently acknowledged by its response from a target.
> Therefore, UDP for iSCSI is a better choice.  NFS is implemented on UDP.
>
> The real question is how many datagrams can we pipe to a receiving node
> without causing overrun.  This issue is being addressed by the number of
> receiving buffers inside the NIC adapter that moves data directly to the
> buffers of an application (known as remote DMA in VI for InfiniBand, or
> physical writes in 1394, or exchange handling in fibre channel.)  It is a
> challenge for people who design the NIC adapter hardware which must keep up
> with the speed of connecting media at two gigabits per second today and 10
> gigabits next year.
>
> The TCP memory to memory copy speed is irrelevant if we have a NIC adapter
> that can transfer data directly to buffers of application software.  If we
> don't have such an adapter, it is an impossible task in trying to define a
> protocol regulating hundreds of SCSI target devices returning data to a
> single SCSI initiator at the same time.  If we slow it down by allowing only
> one SCSI target to return data at a time, then, with the long latency time
> between two nodes, iSCSI does not stand a chance in the world of OC-192,
> gigabit Ethernet, Fibre Channel, InfiniBand, and even 1394B.

-Matt