RE: Avoiding deadlock in iSCSI

["Prasenjit Sarkar/Almaden/IBM" <psarkar@almaden.ibm.com>]

Unfortunately, "implementation issues" determine the complexity of a
protocol and
consequently, whether the benefits of the protocol are worth the cost.

BTW, zero-copy TCP/IP stacks have a lot of caveats (e.g. memory alignment
etc) which
is why they have never made it to any operating system. There are rough
implementations
in Solaris, BSD and Linux but none of them are particularly close to being
robust
(I have tried them all).

What we need is a rational cost-benfit analysis, not simply whether its
just another
implementation issue.

Prasenjit




   Prasenjit Sarkar
   Research Staff Member
   IBM Almaden Research
   San Jose


Michael Krause <krause@cup.hp.com> on 09/17/2000 07:09:51 AM

To:   Prasenjit Sarkar/Almaden/IBM@IBMUS, Robert Snively
      <rsnively@Brocade.COM>
cc:   Kalman Meth/Haifa/IBM@IBMIL, Pierre Labat <pierre_labat@hp.com>,
      ips@ece.cmu.edu
Subject:  RE: Avoiding deadlock in iSCSI



At 09:44 PM 9/16/00 -0700, Prasenjit Sarkar/Almaden/IBM wrote:


>Actually, because of interrupt coalescing in Gigabit Ethernet adapters,
>you can have potentially close many SCSI transactions with one interrupt.
>
>However, you have hit the nail on the head, parallelism tends to (I'm not
>saying will always) increase the average number of interrupts per
>transaction if the parallelism decreases the possibility of interrupt
>coalescing.
>
>And since interrupt coalescing is a statically determined parameter (in
>current implementations), getting speedups out of parallelism is harder
>than it appears.

Interrupt management and parallelism are critical to actual application
throughput.  There are existing solutions that allow throughput to be
achieved with intelligent interrupt management.  I do not believe iSCSI
changes anything along these lines and consider this issue to be
implementation specific.

Mike