On 10/11/15 15:08, Grant Likely wrote:
On Tue, Nov 10, 2015 at 3:04 PM, Zoltan Kiss <zoltan.k...@linaro.org> wrote:
On 10/11/15 12:04, Grant Likely wrote:
On Tue, Nov 10, 2015 at 11:08 AM, Maxim Uvarov <maxim.uva...@linaro.org>
wrote:
On 10 November 2015 at 13:41, Zoltan Kiss <zoltan.k...@linaro.org> wrote:
On 10/11/15 07:39, Maxim Uvarov wrote:
And it looks like it's problem in OVS, not in ODP. I.e. OVS should
allow
to use library functions for fast path (where inlines are critical).
I.e. not just call odp_packet_len(), but move hole OVS function to
dynamic library.
I'm not sure I get your point here, but OVS allows to use dynamic
library
functions on fast path. The problem is that it's slow, because of the
function call overhead.
I'm not familiar with ovs code. But for example ovs has something like:
ovs_get_and_packet_process()
{
// here you use some inlines:
pkt = odp_recv();
len = odp_packet_len(pkt);
... etc.
}
So it's clear for each target arch you needs it's own variant of
ovs_get_and_packet_process() function. That function should go from ovs
to
dynamic library.
Which library? A library specific to OVS? Or some common ODP library
that everyone uses? In either case the solution is not scalable. In
the first case it still requires the app vendor to have a separate
build for each and every supported target. In the second, it is
basically argues for all fast-path application-specific code to go
into a non-app-specific library. That really won't fly.
I have two answers to this question. One for the short term, and one
for the long.
In the short term we have no choice. If we're going to support
portable application binaries, then we cannot do inlines. ODP simply
isn't set up to support that. Portable binaries will have to take the
hit of doing a function call each and every time. It's not fast, but
it *works*, which at least will set a lowest common denominator. To
mitigate the problem we could encourage application packages to
include a generic version (no-inlines, but works everywhere) plus one
or more optimized builds (with inlines) and the correct binary is
selected at runtime. Not great, but it is a reasonable answer for the
short term.
I would argue for the short term to produce platform specific packages as
well, at least for ODP-OVS. As ODP-OVS is not upstream, we need to produce
an openvswitch-odp package anyway (which would set to conflict with the
normal openvswitch package). My idea is to create openvswitch-odp-[platform]
packages, though I don't know if you can set a wildcard conflict rule during
packaging to make sure only one of them are installed at a time.
For the long term to get away from per-platform builds, I see two
viable options. Bill suggested the first: Use LLVM to optimize at
runtime so that thing like inlines get picked up when linked to the
platform library. There is some precedence of other projects already
doing this, so this isn't as far fetched as it may seem.
But wouldn't it tie us down with LLVM?
Does that worry you?
Only that then we require our applications to use LLVM if they want
performance. I don't know the impact of that.
LLVM is a mature project, open source, and lots
of momentum behind it. There are worse things we can do than align
with LLVM when it brings capability that we cannot get anywhere else.
g.
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