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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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 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.

~Andrew

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