linux-kernel - Re: [PATCH v2] 9p/xen: increase XEN_9PFS_RING

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Date:   Mon, 1 Jun 2020 14:06:24 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Dominique Martinet <asmadeus@...ewreck.org>
cc:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        boris.ostrovsky@...cle.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        ericvh@...il.com, lucho@...kov.net,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [PATCH v2] 9p/xen: increase XEN_9PFS_RING_ORDER

n Fri, 22 May 2020, Dominique Martinet wrote:
> Stefano Stabellini wrote on Thu, May 21, 2020:
> > From: Stefano Stabellini <stefano.stabellini@...inx.com>
> > 
> > Increase XEN_9PFS_RING_ORDER to 9 for performance reason. Order 9 is the
> > max allowed by the protocol.
> > 
> > We can't assume that all backends will support order 9. The xenstore
> > property max-ring-page-order specifies the max order supported by the
> > backend. We'll use max-ring-page-order for the size of the ring.
> > 
> > This means that the size of the ring is not static
> > (XEN_FLEX_RING_SIZE(9)) anymore. Change XEN_9PFS_RING_SIZE to take an
> > argument and base the calculation on the order chosen at setup time.
> > 
> > Finally, modify p9_xen_trans.maxsize to be divided by 4 compared to the
> > original value. We need to divide it by 2 because we have two rings
> > coming off the same order allocation: the in and out rings. This was a
> > mistake in the original code. Also divide it further by 2 because we
> > don't want a single request/reply to fill up the entire ring. There can
> > be multiple requests/replies outstanding at any given time and if we use
> > the full ring with one, we risk forcing the backend to wait for the
> > client to read back more replies before continuing, which is not
> > performant.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...inx.com>
> 
> LGTM, I'll try to find some time to test this by the end of next week or
> will trust you if I can't make it -- ping me around June 1st if I don't
> reply again until then...

Ping :-)