Re: Merging relayfs?

[Tom Zanussi <zanussi@us.ibm.com>]

Steven Rostedt writes:
 > On Sun, 2005-07-17 at 10:52 -0500, Tom Zanussi wrote:
 > 
 > >  > 
 > >  > >  > - overwrite mode can be implemented via the buffer switch callback
 > >  > > 
 > >  > > The buffer switch callback is already where this is handled, unless
 > >  > > you're thinking of something else - one of the first checks in the
 > >  > > buffer switch is relay_buf_full(), which always returns 0 if the
 > >  > > buffer is in overwrite mode.
 > >  > 
 > >  > I mean, relayfs doesn't has to know about this, the client itself can do 
 > >  > it (e.g. via helper functions).
 > > 
 > > In a previous version, we did something like having the client pass
 > > back a return value from the callback indicating whether or not to
 > > continue or stop.  I can try doing something like that instead again.
 > 
 > Tom,
 > 
 > I'm actually very much against this. Looking at a point of view from the
 > logdev device. Having a callback to know to continue at every buffer
 > switch would just be slowing down something that is expected to be very
 > fast. I don't see the problem with having an overwrite mode or not. Why
 > can't relayfs know this? It _is_ an operation of relayfs, and having it
 > pushed to the client would seem to make the client need to know more
 > about how relayfs works that it needs to.  Because, the logdev device
 > doesn't care about buffer switches.

I don't think it would slow anything down - it would be pretty much
the same code being executed as before e.g. the buffer_start()
callback for overwrite mode could look like this:

int buffer_start()
{
	...	
	return 1; // continue unconditionally
}

And for no-overwrite mode:

int buffer_start()
{
	...	
	return !relay_buf_full(buf); // continue if not full
}

Since the buffer start callback already returns the amount that's
supposed to be reserved at the start of the sub-buffer, I'd have to
make that an outparam instead, I guess, but it's basically the same
code handling the overwrite/no-overwrite condition.

Tom