linux-kernel - Re: kdbus: to merge or not to merge?

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Message-ID: <CALCETrW4YcVAuGZb=kCR_NmTQnMtE+LMGFtPpdszSyHrQp42mA@mail.gmail.com>
Date:	Fri, 7 Aug 2015 08:09:31 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Daniel Mack <daniel@...que.org>
Cc:	David Herrmann <dh.herrmann@...il.com>,
	Tom Gundersen <teg@...m.no>,
	"Kalle A. Sandstrom" <ksandstr@....fi>,
	Greg KH <gregkh@...uxfoundation.org>,
	Borislav Petkov <bp@...en8.de>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Havoc Pennington <havoc.pennington@...il.com>,
	Djalal Harouni <tixxdz@...ndz.org>, cee1 <fykcee1@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: kdbus: to merge or not to merge?

On Fri, Aug 7, 2015 at 7:40 AM, Daniel Mack <daniel@...que.org> wrote:
> On 08/06/2015 08:43 PM, Andy Lutomirski wrote:
>> Nonetheless, it looks like the slice logic (aside: it looks *way* more
>> complicated than necessary -- what's wrong with circular buffers)
>> will, under most (but not all!) workloads, concentrate access to a
>> smallish fraction of the pool.  This is IMO bad, since it means that
>> most of the time most of the pool will remain uncommitted.  If, at
>> some point, something causes the access pattern to change and hit all
>> the pages (even just once), suddenly all of the pools get committed,
>> and your memory usage blows up.
>
> That's a general problem with memory overcommitment, and not specific to
> kdbus. IOW: You'd have the same problem with a similar logic implemented
> in userspace, right?
>

Sure, except that, if it's in userspace and it starts causing
problems, then userspace can fix it without running into kernel ABI
stability issues.

--Andy
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