Date:	Tue, 04 Aug 2009 14:44:37 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 0/2] new kfifo API

Am Dienstag, den 04.08.2009, 14:24 +0200 schrieb Arnd Bergmann:
> On Monday 03 August 2009, Stefani Seibold wrote:
> > Am Montag, den 03.08.2009, 21:00 +0200 schrieb Arnd Bergmann:
> > 
> > DECLARE_KFIFO looks for me more useful, because i can use it inside a
> > struct decalaration. And then i need INIT_KFIFO for initializing this.
> > 
> > BTW: DECLARE_...., DEFINE_..... and INIT_..... are linux style. Habe a
> > look at workqueue.h, wait.h, types.h, semaphore.h, rwsem-spinlock.h,
> > interrupt.h, completion.h, seqlock.h and so on....
> 
> Yes, you are right. I realized that myself after I sent out my
> comments.
> 
> > > 1. you can no longer use preallocated buffers, which limits the possible
> > > users to those that are unrestricted to the type of allocation.
> > > 2. The size of the buffer is no longer power-of-two. In fact, it's guaranteed
> > > to be non-power-of-two because kmalloc gives you a power-of-two allocation
> > > but now you also put the struct kfifo in there.
> > > 
> > > Users that need a power-of-two buffer (the common case) now waste almost
> > > 50% of the space.
> > > 
> > 
> > Okay, give me a thought about this....... yes you are right ;-( But what
> > is with vmalloc? 128 MB should be enough?
> 
> vmalloc also has performance problems on some architectures that can
> access the linear mapping faster than paged memory and it is
> rather wasteful if you have 64kb pages.
> 
> I don't think the total size matters, the 128 MB limit only exists
> if you have a 32 bit CPU and 1GB or more of memory, which is hopefully
> getting rarer and already causes other problems (highmem...).
> 
> kmalloc currently limits the kfifo size to something like 128kb (arch
> specific), if you need more than that, you need alloc_pages(), which
> is limited to a power-of-two amount of pages.
> 

You are right, i don't like vmalloc too. It was only a first thought ;-)

> > > The requirement for power-of-two also meant a much faster __kfifo_off
> > > function on certain embedded platforms that don't have an integer division
> > > instruction in hardware.
> >
> > Yes i know this argument, but since the day of the 6502 and Z80 i have
> > never seen this kind of CPU. Okay i forgot to mention the stupid ARM
> > CPU, but newer ARM cores have a hardware division support.
> 
> I think this is actually more relevant than the vmalloc limit you mentioned,
> demand for tiny processors will probably stay because of cost reasons.
> Architectures that we support in Linux without integer divide include
> arm, blackfin, h8300, ia64 (!), m68k, microblaze, sh and xtensa.
> 

I'm embedded developer too... so i know what you mean. But for the fifo
this is not really a problem, managing and copying the data will be the
bigger amount. But with my new version i go back to the old power of two
method.

> Your first version with the non-power-of-two buffers also had a bug
> in the handling because it would not handle 32 bit integer overflows
> correctly. To get those right, you need an extra branch every time you
> add to the counter.
> 

Ooops, that was a real bug... Thanks.

> Your second version is ok in this regard because it uses the original
> size logic.

Does it mean you like it now ;-) ???? I think we are on a good way!

> 
> 	Arnd <><

Stefani <\_,


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