linux-kernel - Re: [RFC -v2] kfifo writer side lock-less support

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Date:	Wed, 25 Aug 2010 08:38:47 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	"huang.ying.caritas@...il.com" <huang.ying.caritas@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC -v2] kfifo writer side lock-less support

On Wed, 2010-08-25 at 03:13 +0800, Stefani Seibold wrote:
> > > > The patch adds only 1 field (unsigned int) to struct __kfifo. I think
> > > > that should be acceptable. Because sizeof(struct __kfifo) should be much
> > > > smaller that __kfifo->mask + 1 in most cases.
> > > 
> > > I don't know what you mean with "because sizeof(struct __kfifo) should
> > > be much smaller that __kfifo->mask + 1 in most cases". I am convinced
> > > that you did not really understand the kfifo code. sizeof(struct
> > > __kfifo) is constant and __kfifo->mask + 1 is the fifo size in elements,
> > > which is not constant. Before you answering study the code first!
> > > 
> > > And is not acceptable to bload the struct __kfifo, because it will never
> > > need by the most users.
> > 
> > I mean, for most user, __kfifo->mask + 1 > sizeof(struct __kfifo), so
> > another 4 bytes for each user is relatively small.
> > 
> 
> You have no idea. As i wrote you should study the code before answering!
> 
> sizeof(struct __kfifo) is always 20 bytes on a 32 bit cpu, and
> kfifo->mask +1 depends on the size of the number of fifo elements and it
> is an initialization parameter. 

After my changing, sizeof(struct __kfifo) should be 24 on 32 bit CPU,
that is 4 bytes more. But I think for most users, kfifo->mask + 1 should
be hundreds or thousands. If the average(kfifo->mask + 1) = 256, the
increment percentage for each user is about:

4 / (256 + 20) = 1.45%

So I think the changes to the size of struct __kfifo should be
acceptable.

Best Regards,
Huang Ying


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