Re: [RFC][PATCH] Cross Memory Attach

[Ingo Molnar]

* Avi Kivity <avi@xxxxxxxxxx> 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.

Thanks,

	Ingo
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