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Andre van der Vlies (A.G.van.der.Vlies@cb.hva.nl)
Tue, 8 Apr 1997 10:33:55 +0200 (CDT) 

Date: Tue, 8 Apr 1997 10:33:55 +0200 (CDT)
From: Andre van der Vlies <A.G.van.der.Vlies@cb.hva.nl>
Subject: Re: Fw: [mSQL] Locking?
Message-Id: <Pine.SGI.3.94.970408100356.21968H-100000@ice.cb.hva.nl>

On Sun, 6 Apr 1997, Mark-Jason Dominus wrote:

> On Sun, 6 Apr 1997, Clyde Smith-Stubbs wrote:
> > Oh, the best way to requery would be to wait a few seconds, then requery.
> > It should not impact performance much if you don't requery too
> > often.
>
> And because your process can tell how far back in the queue it is, it
> can compute a poisson distribution to desice how long to wait. If
> there are 100 other processes ahead of it, it should sleep longer than
> if it is next to go.
>

This is disturbing info. 'INSERT' and 'UPDATE' aren't atomic operations??
What about implementing Dijkstra's semaphores (using SYSV ipc) on this? This
shouldn't be to hard to do ...

        #define P(x) work_sema(x, -1)
                #define V(x) work_sema(x, 1)

                #include <sys/sem.h>

                int lock;
                mqsl_lock = semget(IPC_PRIVATE, 1, 0600);
                semctl(mqsl_lock, 1, SETVAL, 1);

        work_sema(int sem, int action) {

                        struct sembuf semb;
                                                                                
                        semb.sem_num = sem;
                        semb.sem_op = action;
                        semb.sem_flg = 0;
                        semop(semid, &semb, 1);
                }

... find the critical section for 'UPDATE' and 'INSERT' and surround it with
P(msql_lock) and V(msql_lock). There is a drawback; if msql crashes the semaphores
must be removed manualy ... And some code for removal on sigint and such has
to be included.

If got some spare time I will have a look into that.

                Greetings,
                           Andre van der Vlies <A.G.van.der.Vlies@cb.hva.nl>
                           System Administrator
                           Hogeschool van Amsterdam
                           Stadhouderskade 55, Amsterdam
                           tel: (+31) 20 5702670
                           fax: (+31) 20 5702510
                           http://www.cb.hva.nl/~andre
Key fingerprint = 99 DC D6 01 96 C2 48 80 DC 9C D5 D2 34 F6 A6 5D

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