----- Original Message -----
From: "Jan Hudec" <bulb@ucw.cz>
To: "Magnus Myrefors" <tkv764x@tninet.se>
Cc: <kernelnewbies@nl.linux.org>
Sent: Tuesday, July 15, 2003 8:28 PM
Subject: Re: Semaphores in multiprocessor systems.
> On Tue, Jul 15, 2003 at 03:08:16PM +0200, Magnus Myrefors wrote:
> > I wonder if it is considered to be a multiprocessor-system in linux if
> > you install linux on two pc's and connect them with ethernet or a bus
> > ?
>
> No, it's called a cluster. In a wider sense it is a multiprocessor
> system, but I suppose you wanted to say "SMP system", which it is not.
>
> SMP (Symetric MultiProcessing) system is a system, that contains several
> processors connected to a common bus and sharing a single memory unit.
>
> Note: You _can't_ connect two computers with a local bus.
>
> > If it is, how do you synchronize access to a memoryunit also connected
> > with the two pc's with semaphores?
>
> They don't have a common memory unit. Only way processes can communicate
> is by sending each other network packets.
>
> > Does this type of programming lie within linux?
>
> No, Linux is not a distributed system.
>
> > If it isn't I suppose you could solve it something
> > like this:
>
> > I guess you cannot have the code for the semaphore compiled on the
> > memoryunit since the two pc's may have different
> > hardware-structures. So, instead I believe you can have
> > a memoryaddress on the memoryunit to protect some codearea. The
> > address initially set to 1 and decreased when one pc wants to
> > access the codearea and set back to 1 when finished.
>
> Not that I really know what you wan't to do, but I really fear it won't
> work for the case you want.
>
> This will work for *LOCAL* lock implementation:
> - You have a shared variable initialized to 1.
> - Process wanting to lock does:
> 1 Atomic dec
> 2 Check if it's zero
> 3 If it is zero: locked ok.
> 4 Else atomic inc and goto 1.
> - Unlocking is just atomic inc.
> Problem is with the "atomic" - you can't get that on distributed system.
>
> So for a distributed system you need some distributed locking mechanizm.
> There is one implemented in any library for distributed computations.
> Probably best such library is LAM/MPI.
>
Continuing on the local approach:
(In the case when you are about to send data between pc1 and pc2 using
a bus
and don't use regular packages)
Then if you use a single test to check if it's ok for the receiving pc
to start reading
the data protected by the "spin lock" type of semaphor it should be
possible to loose
data if the sending pc works faster than the receiving.
To avoid that you could number the chunks of data in an ascending manner
and perhaps
using some of the bandwith of the bus to exchange information about the
current chunk-number.
You may also want to use two variables to check, one that is set to 1
(by the sender) if the sender is
accessing the data to be transmitted and 0 if the data is avaible and
the other set to 1 (by the receiver)
if the receiver is accessing the data and to 0 if the data is avaible.
So the receiving pc does this:
(*)Check if it's ok to read data (by checking the variable
"changing_data") AND compare
"chunk_number_read with "current_chunk_number" (should not be
equal).
If the checks are ok:
Set the variable "reading_data" to 1.
Read the data
Increase the variabel "chunk_number_read".
Set variabel "reading_data" back to 0
If they are not:
Do nothing (wait and check (*) again )
The sending pc does this:
(**)Check the variable "reading_data" AND compare "chunk_number_read"
with
the sender's "current_chunknumber" ("current_chunknumber" should
be one over
"chunk_number_read").
If the checks are ok:
Set variable "changing_data" to 1.
Change the data
Set "changing_data" back to 0
If they are not:
Do nothing (wait and check (**) again)
Magnus
> --------------------------------------------------------------------------
-----
> Jan 'Bulb' Hudec <bulb@ucw.cz>
> --
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>
>
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