Re: [PATCH v2 2/4] NFSD: Add READ_PLUS support for data segments

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, Feb 06, 2015 at 03:54:56AM -0800, Christoph Hellwig wrote:
> On Thu, Feb 05, 2015 at 11:43:46AM -0500, Anna Schumaker wrote:
> > > The problem is that the typical case of all data won't use splice
> > > every with your patches as the 4.2 client will always send a READ_PLUS.
> > > 
> > > So we'll have to find a way to use it where it helps.  While we might be
> > > able to add some hacks to only use splice for the first segment I guess
> > > we just need to make the splice support generic enough in the long run.
> > > 
> > 
> > I should be able to use splice if I detect that we're only returning a single DATA segment easily enough.
> 
> You could also elect to never return more than one data segment as a
> start:
> 
>    In all situations, the
>    server may choose to return fewer bytes than specified by the client.
>    The client needs to check for this condition and handle the
>    condition appropriately.

Yeah, I think that was more-or-less what Anna's first attempt did and I
said "what if that means more round trips"?  The client can't anticipate
the short reads so it can't make up for this with parallelism.

> But doing any of these for a call that's really just an optimization
> soudns odd.  I'd really like to see an evaluation of the READ_PLUS
> impact on various workloads before offering it.

Yes, unfortunately I don't see a way to make this just an obvious win.

(Is there any way we could make it so with better protocol?  Maybe RDMA
could help get the alignment right in multiple-segment cases?  But then
I think there needs to be some sort of language about RDMA, or else
we're stuck with:

	https://tools.ietf.org/html/rfc5667#section-5

which I think forces us to return READ_PLUS data inline, another
possible READ_PLUS regression.)

Anyway, ignoring RDMA, I guess the interesting questions to me are:

	- How much does READ_PLUS actually help in the cases it was
	  meant to?  Perhaps create and age a VM image somehow, and then
	  time a read of the entire image?
	- What are the worst-case regressions?  Read a large file with a
	  lot of small holes and measure time and cpu usage?

--b.