Re: [RFC contig pages support 1/2] IB: Supports contiguous memory operations

[Vlastimil Babka <vbabka@suse.cz>]

On 12/13/2015 01:48 PM, Shachar Raindel wrote:
>
>
>> -----Original Message-----
>> From: Christoph Hellwig [mailto:hch@infradead.org]
>> Sent: Wednesday, December 09, 2015 8:40 PM
>>
>> On Wed, Dec 09, 2015 at 10:00:02AM +0000, Shachar Raindel wrote:
>>> As far as gain is concerned, we are seeing gains in two cases here:
>>> 1. If the system has lots of non-fragmented, free memory, you can
>> create large contig blocks that are above the CPU huge page size.
>>> 2. If the system memory is very fragmented, you cannot allocate huge
>> pages. However, an API that allows you to create small (i.e. 64KB,
>> 128KB, etc.) contig blocks reduces the load on the HW page tables and
>> caches.
>>
>> None of that is a uniqueue requirement for the mlx4 devices.  Again,
>> please work with the memory management folks to address your
>> requirements in a generic way!
>
> I completely agree, and this RFC was sent in order to start discussion
> on this subject.
>
> Dear MM people, can you please advise on the subject?
>
> Multiple HW vendors, from different fields, ranging between embedded SoC
> devices (TI) and HPC (Mellanox) are looking for a solution to allocate
> blocks of contiguous memory to user space applications, without using huge
> pages.
>
> What should be the API to expose such feature?
>
> Should we create a virtual FS that allows the user to create "files"
> representing memory allocations, and define the contiguous level we
> attempt to allocate using folders (similar to hugetlbfs)?
>
> Should we patch hugetlbfs to allow allocation of contiguous memory chunks,
> without creating larger memory mapping in the CPU page tables?
>
> Should we create a special "allocator" virtual device, that will hand out
> memory in contiguous chunks via a call to mmap with an FD connected to the
> device?

How much memory do you assume to be used like this? Is this memory 
supposed to be swappable, migratable, etc? I.e. on LRU lists?
Allocating a lot of memory (e.g. most of userspace memory) that's not 
LRU wouldn't be nice. But LRU operations are not prepared to work witch 
such non-standard-sized allocations, regardless of what API you use.  So 
I think that's the more fundamental questions here.

> Thanks,
> --Shachar
>
>
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