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SubjectRe: [RFC][PATCH] Cross Memory Attach
  On 09/15/2010 03:51 PM, Ingo Molnar wrote:
> * Avi Kivity<> wrote:
>> On 09/15/2010 03:18 AM, Christopher Yeoh wrote:
>>> The basic idea behind cross memory attach is to allow MPI programs
>>> doing intra-node communication to do a single copy of the message
>>> rather than a double copy of the message via shared memory.
>> If the host has a dma engine (many modern ones do) you can reduce this
>> to zero copies (at least, zero processor copies).
>>> The following patch attempts to achieve this by allowing a
>>> destination process, given an address and size from a source
>>> process, to copy memory directly from the source process into its
>>> own address space via a system call. There is also a symmetrical
>>> ability to copy from the current process's address space into a
>>> destination process's address space.
>> Instead of those two syscalls, how about a vmfd(pid_t pid, ulong
>> start, ulong len) system call which returns an file descriptor that
>> represents a portion of the process address space. You can then use
>> preadv() and pwritev() to copy memory, and io_submit(IO_CMD_PREADV)
>> and io_submit(IO_CMD_PWRITEV) for asynchronous variants (especially
>> useful with a dma engine, since that adds latency).
>> With some care (and use of mmu_notifiers) you can even mmap() your
>> vmfd and access remote process memory directly.
>> A nice property of file descriptors is that you can pass them around
>> securely via SCM_RIGHTS. So a process can create a window into its
>> address space and pass it to other processes.
>> (or you could just use a shared memory object and pass it around)
> Interesting, but how will that work in a scalable way with lots of
> non-thread tasks?
> Say we have 100 processes. We'd have to have 100 fd's - each has to be
> passed to a new worker process.
> In that sense a PID is just as good of a reference as an fd - it can be
> looked up lockless, etc. - but has the added advantage that it can be
> passed along just by number.

It also has better life-cycle control (with just a pid, you never know
what it refers to unless you're its parent). Would have been better if
clone() returned an fd from which you could derive the pid if you wanted
to present it to the user.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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