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Message-ID: <cbf3e752-8f7e-4e74-a709-8b812bdc36c0@lunn.ch>
Date: Wed, 5 Jun 2024 02:52:29 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Paolo Abeni <pabeni@...hat.com>,
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Subject: Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator
> How is the compiler going to know which path is going to be taken the most?
> There's two main paths in the ring buffer logic. One when an event stays on
> the sub-buffer, the other when the event crosses over to a new sub buffer.
> As there's 100s of events that happen on the same sub-buffer for every one
> time there's a cross over, I optimized the paths that stayed on the
> sub-buffer, which caused the time for those events to go from 250ns down to
> 150 ns!. That's a 40% speed up.
>
> I added the unlikely/likely and 'always_inline' and 'noinline' paths to
> make sure the "staying on the buffer" path was always the hot path, and
> keeping it tight in cache.
>
> How is a compiler going to know that?
It might have some heuristics to try to guess unlikely/likely, but
that is not what we are talking about here.
How much difference did 'always_inline' and 'noinline' make? Hopefully
the likely is enough of a clue it should prefer to inline whatever is
in that branch, where as for the unlikely case it can do a function
call.
But compilers is not my thing, which is why i would reach out to the
compiler people and ask them, is it expected to get this wrong, could
it be made better?
Andrew
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