Re: [OMPI devel] [PATCH v5 0/4] vm: add a syscall to map a process memory into a pipe

On Mon, Feb 26, 2018 at 09:38:19AM -0700, Nathan Hjelm wrote:

All MPI implementations have support for using CMA to transfer data
between local processes. The performance is fairly good (not as good as
XPMEM) but the interface limits what we can do with to remote process
memory (no atomics). I have not heard about this new proposal. What is
the benefit of the proposed calls over the existing calls?

The proposed system call call that combines functionality of
process_vm_read and vmsplice [1] and it's particularly useful when one
needs to read the remote process memory and then write it to a file
descriptor. In this case a sequence of process_vm_read() + write() calls
that involves two copies of data can be replaced with process_vm_splice() +
splice() which does not involve copy at all.

[1] https://lkml.org/lkml/2018/1/9/32
 

-Nathan

quoted

On Feb 26, 2018, at 2:02 AM, Pavel Emelyanov [off-list ref] wrote:

On 02/21/2018 03:44 AM, Andrew Morton wrote:

quoted

On Tue,  9 Jan 2018 08:30:49 +0200 Mike Rapoport [off-list ref] wrote:

quoted

This patches introduces new process_vmsplice system call that combines
functionality of process_vm_read and vmsplice.

All seems fairly strightforward.  The big question is: do we know that
people will actually use this, and get sufficient value from it to
justify its addition?

Yes, that's what bothers us a lot too :) I've tried to start with finding out if anyone
used the sys_read/write_process_vm() calls, but failed :( Does anybody know how popular
these syscalls are? If its users operate on big amount of memory, they could benefit from
the proposed splice extension.

-- Pavel

-- 
Sincerely yours,
Mike.