linux-kernel - Re: [PATCH 03/15] ring_buffer: Align buffer_page struct allocations only to fit the flags.

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Message-ID: <AANLkTimEcb9k=0_gSH7SdjHg=hywKfNjq=4p7HnUFJFZ@mail.gmail.com>
Date:	Tue, 7 Dec 2010 14:44:01 -0800
From:	David Sharp <dhsharp@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, mrubin@...gle.com
Subject: Re: [PATCH 03/15] ring_buffer: Align buffer_page struct allocations
 only to fit the flags.

On Fri, Dec 3, 2010 at 5:43 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 2010-12-03 at 16:13 -0800, David Sharp wrote:
>> buffer_page structs need to be aligned to 4 byte boundaries because the page
>> flags are stored in the two least-significant bits of the pointers in the page
>> list. Aligning to cache lines is sufficient, but doesn't seem to be necessary.
>> Reducing the alignement to only 4 bytes may improve cache efficiency.
>
> The reason for the cache line boundaries because the bpages are per cpu,
> and if they are allocated next to some variable that gets change
> globally or even another bpage that is used for another cpu buffer, then
> I would expect we would get some cache line bouncing.

I see. That makes sense. I'm not attached to this patch, we can drop it.

> Perhaps we should make bpage have its own slab allocation
> (kmem_cache_alloc). And perhaps even try to keep all bpages that share
> the same cache line on the same cpu buffer.

Maybe. Although, I was thinking of some patches to keep pages in a
free-pool, so that when a cpu buffer needs a new page, it takes from
the pool, and when a reader is done with a page, it returns to the
free-pool. (This helps with the situation where more events occur on
one CPU than others: pages are not locked to a CPU that isn't
producing events.) In that case, it would be impossible to predict
what cpu a bpage belongs to. But since this is theoretical, maybe it's
not relevant.
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