Re: [RFC PATCH] ptr_ring: linked list fallback

On Fri, Feb 16, 2018 at 04:32:05PM -0500, David Miller wrote:

From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Fri, 16 Feb 2018 09:40:54 +0200

quoted

So pointer rings work fine, but they have a problem:
make them too small and not enough entries fit.
Make them too large and you start flushing your cache
and running out of memory.

This is a new idea of mine: a ring backed by a
linked list. Once you run out of rin entries,
instead of a drop you fall back on a list with
a common lock.

Should work well for the case where the ring is typically sized
correctly, but will help address the fact that some user try to set e.g.
tx queue length to 1000000.

My hope this will move us closer to direction where e.g. fw codel can
use ptr rings without locking at all.
The API is still very rough, and I really need to take a hard look
at lock nesting.

Completely untested, sending for early feedback/flames.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

So the idea is that if a user sets a really huge TX queue length, we allocate
a ptr_ring which is smaller, and use the backup linked list when necessary
to provide the requested TX queue length legitimately.

Right?

Exactly, thanks for adding this clarification.

-- 
MST