[TBD]How to solve Multiple reader/single writer problem

Introduction

Many times, we see that there are multiple readers wanting to get the values and few writers who changes the value. For example, in a firewall policy, there will be occasional change of policy. However, policy will be evaluated whenever a data packet arrives.

Similarly for a website showing news, reading will happen more often than writing news. In such cases, it is more important to give some advantage to read operation over write.

Implementation using Semaphore

Shared variables: Semaphore mutex, wrl;

integer rcount;

Init: mutex = 1, wrl = 1, rcount = 0;

Writer

do {

P(wrl);

. . .

/*writing is performed*/

. . .

V(wrl);

}while(TRUE);

Reader

do {

P(mutex);

rcount++;

if (rcount == 1)

P(wrl);

V(mutex);

. . .

/*reading is performed*/

. . .

P(mutex);

rcount--;

if (rcount == 0)

V(wrl);

V(mutex);

}while(TRUE);

Implementation using conditional variable

The lock-for-read operation in this setup is:^[6][7][8]

The lock-for-write operation is similar, but slightly different (inputs are the same as for lock-for-read):^[6][7][8]

Reference

https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock

http://heather.cs.ucdavis.edu/~matloff/158/PLN/RWLock.c

www.cs.cornell.edu/courses/cs414/2007sp/lectures/09-syncproblems.ppt

http://heather.cs.ucdavis.edu/~matloff/158/PLN/RWLock.c

https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock