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Re: [Xen-devel] [PATCH] oxenstored: fix short-write issue

On Mon, 2015-11-02 at 14:10 +0000, Andrew Cooper wrote:
> On 02/11/15 13:44, Ian Campbell wrote:
> > On Tue, 2015-10-27 at 17:10 +0000, Wei Liu wrote:
> > > When oxenstored wrote to the ring, it wrote a chunk of contiguous
> > > data.
> > > Originally when it tried to write across ring boundary, it returned a
> > > short-write when there is still room.  That led to stalling mini-os's
> > > xenstore thread at times.
> > What is a "short-write" in this context?
> > 
> > Given data bytes 0..M I assumed it is only writing bytes 0..N and not
> > N+1..M because the ring boundary is at N. But what is it writing to the
> > ->prod ring pointer N or M?
> Prod gets incremented by N in this case.
> > 
> > AIUI writing N should be allowed by the ring protocol, the client
> > should
> > keep looking for more data until it has a complete request.
> > 
> > Writing M would be a server error.
> Correct, and this is what is happening.

The first or second? Your first comment suggests the first, but your second
binds most closely to the second.

> The server (believes) that the ring is full, when it is not.  It waits
> for the client to make more space in the ring, while the client is
> waiting for the server to complete its message in the ring, thus
> stalling.

That makes sense, thanks.

I think this needs to be spelled out more fully in the commit message, in
particular "server thinks ring is full when it is not" is the most relevant
thing, the short write is just how we arrived there.


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