Re: [OSS Proxy] [osspd:bugs] #4 alsap output too fast

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Hey, Miklos.

On Tue, Feb 05, 2013 at 03:41:24PM +0100, Miklos Szeredi wrote:
> So basically my patch did a custom client side mmap and Linus said
> that that could be better implemented with shmem.  Server side mmap
> wasn't involved, that's the part I didn't like about your original
> approach, so a server side data store/retrieve was added instead and
> osspd modified to use that.
> 
> I couldn't find anything wrong with what Linus said.  Of course it may
> turn out that there's a reason why it won't work, but it doesn't sound
> too difficult to implement.

Ah, okay, it's about the client side of mmap.

> > So, I was talking with Avati about gluster the other day, which makes
> > pretty heavy use of FUSE and reportedly hits bottleneck due to the
> > double data copy on high bandwidth configurations.  We were talking
> > about ways to reduce data copy.  The only obvious thing to do is
> > mapping the page cache pages into the server address space for both
> > page-cached backed and O_DIRECT files, so at least we do have a much
> > stronger use case for mmapping pages into server address space.  If
> > direct mmapping is the way to go, I can dust off my old patches.  What
> > do you think?
> 
> I think the idea to share pages through memory maps is fundamentally
> flawed.  Doing zero copy with splice is saner, I think, and there's a

It is a bit cumbersome but I can't see why it would be fundamentally
flawed.  That said, yeap, I agree splice would be a lot nicer.

> lot of infrastructure already there in fuse.  And in theory nothing
> prevents improving that and achieving true zero copy with direct-IO
> and single-copy with buffered-IO.  BTW you couldn't do server side
> mapping of O_DIRECT anyway since no page cache is involved with that.

IIRC, the server side created a file serving as backing store for the
direct mmap area.  quake could play sound through it, so it worked.
One way or the other, we need to provide an address_space.

Thanks.

-- 
tejun