Messages in this thread |  | | Date | Sun, 16 Nov 2008 13:51:30 -0800 | From | Arjan van de Ven <> | Subject | Re: [PATCH 1/7] mm: introduce simple_malloc()/simple_free() |
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On Mon, 17 Nov 2008 07:39:55 +1000 "Dave Airlie" <airlied@gmail.com> wrote:
> On Mon, Nov 17, 2008 at 4:57 AM, Arjan van de Ven > <arjan@infradead.org> wrote: > > On Sun, 16 Nov 2008 00:19:26 -0800 (PST) > > David Miller <davem@davemloft.net> wrote: > > > >> From: Arjan van de Ven <arjan@infradead.org> > >> Date: Sat, 15 Nov 2008 20:52:29 -0800 > >> > >> > On Sun, 16 Nov 2008 12:33:15 +0800 > >> > Lai Jiangshan <laijs@cn.fujitsu.com> wrote: > >> > > >> > > > >> > > some subsystem needs vmalloc() when required memory is large. > >> > > but current kernel has not APIs for this requirement. > >> > > this patch introduces simple_malloc() and simple_free(). > >> > > >> > I kinda really don't like this approach. vmalloc() (and > >> > especially, vfree()) is a really expensive operation, and > >> > vmalloc()'d memory is also slower (due to tlb pressure). > >> > Realistically, people should try hard to use small datastructure > >> > instead.... > >> > >> This is happening in many places, already, for good reason. > >> > >> There are lots of places where we can't (core hash tables, etc.) > >> and we want NUMA spreading and reliable allocation, and thus > >> vmalloc it is. > > > > vmalloc() isn't 100% evil; for truely long term stuff it's > > sometimes a quite reasonable solution. > > > > There are some issues with it still: the vmalloc() space is shared > > with ioremap, modules and others and it's not all that big on 32 > > bit; on x86 you could well end up with only 64Mb total (after > > taking out the various ioremap's etc). > > > > Yes there's places where it's then totally fine to dip into this > > space at boot/init time. You mention a few very good users. > > (There's still the tlb miss cost on use but on modern cpus a tlb > > miss is actually quite cheap) > > > > But this doesn't make vmalloc() the magic bullet that solves the "oh > > Linux can't allocate large chunks of memory" problem. Specifically > > in driver space for things that get ported from other OSes. > > So we keep the duplicated code? or we just audit new callers.... I > think this patch > makes it easier to spot new callers doing something stupid. As davem > said we duplicate > this code all over the place, so for that reason along a simple > wrapper makes things a lot > easier, and also possibly a lot easier to change in the future to a > new non-sucky API. > > So I'm all for it maybe with a non simple name. >
I would go further than this.
Make the code just use vmalloc(). Period.
But then make vmalloc() smart and try do a direct mapping allocation first, before falling back to a virtual mapping. (and based on size it wouldn't even try it for just big things)
-- Arjan van de Ven Intel Open Source Technology Centre For development, discussion and tips for power savings, visit http://www.lesswatts.org
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