Messages in this thread | | | Date | Wed, 28 Jul 2010 09:08:54 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [RFC][PATCH 1/7][memcg] virtually indexed array library. |
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On Tue, 27 Jul 2010 12:29:49 -0600 Jonathan Corbet <corbet@lwn.net> wrote:
> On Tue, 27 Jul 2010 16:53:03 +0900 > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote: > > > This virt-array allocates a virtally contiguous array via get_vm_area() > > and allows object allocation per an element of array. > > Quick question: this looks a lot like the "flexible array" mechanism > which went in around a year ago, and which is documented in > Documentation/flexible-arrays.txt. I'm not sure we need two of > these... That said, it appears that there are still no users of > flexible arrays. If your virtually-indexed arrays provide something > that flexible arrays don't, perhaps your implementation should replace > flexible arrays?
Hmm. As Documentatin/flexible-arrays.txt says,
"The down sides are that the arrays cannot be indexed directly, individual object size cannot exceed the system page size, and putting data into a flexible array requires a copy operation. "
This virtually-indexed array is
- the arrays can be indexed directly. - individual object size can be defined arbitrary. - puttind data into virt-array requires memory allocation via alloc_varray_item().
But, virtyally-indexed array has its own down side, too.
- It uses vmalloc() area. This can be very limited in 32bit archs. - It cannot be used in !MMU archs. - It may consume much TLBs because vmalloc area tends not to be backed by hugepage.
Especially, I think !MMU case is much different. So, there are functional difference. I implemented this to do quick direct access to objects by indexes. Otherwise, flex-array may be able to provide more generic frameworks.
Then, I myself don't think virt-array is a replacemento for flex-array.
A discussion "flex-array should be dropped or not" is out of my scope, sorry. I think you can ask to drop it just because it's almost dead without mentioning virt-array.
Thanks, -Kame
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