Re: Efficient IPC mechanism on Linux

[Andrea Arcangeli]

On Wed, Sep 10, 2003 at 07:39:17PM +0200, Martin Konold wrote:
> Am Wednesday 10 September 2003 06:59 pm schrieb Andrea Arcangeli:
> 
> Hi,
> 
> > design that I'm suggesting. Obviously lots of apps are already using
> > this design and there's no userspace API simply because that's not
> > needed.
> 
> HPC people have investigated High performance IPC many times basically it 
> boils down to:
> 
> 1. Userspace is much more efficient than kernel space. So efficient 
> implementions avoid kernel space even for message transfers over networks 
> (e.g. DMA directly to userspace). 
> 
> 2. The optimal protocol to use and the number of copies to do is depending on 
> the message size.
> 
> Small messages are most efficiently handled with a single/dual copy (short 
> protocol / eager protocol) and large messages are more efficient with 
> single/zero copy techniques (get protocol) depending if a network is involved 
> or not.
> 
> Traditionally in a networked environment single copy means PIO and zero copy 
> means DMA when using network hardware.
> 
> The idea is while DMA has much higher bandwidth than PIO DMA is more expensive 
> to initiate than PIO. So DMA is only useful for large messages.

agreed.

> 
> In the local SMP case there do exist userspace APIs like MPI which can do 

btw, so far we were only discussing IPC in a local box (UP or SMP or
NUMA) w/o networking involved. Luca's currnet implementation as well was
only working locally.

> single copy Message passing at memory speed in pure userspace since many 
> years.
> 
> The following PDF documents a measurement where the communication between two 
> processes running on different CPUs in an SMP system is exactly the memcpy 
> bandwidth for large messages using a single copy get protocol.
> 
> 	http://ipdps.eece.unm.edu/1999/pc-now/takahash.pdf
> 
> Numbers from a Dual P-II-333, Intel 440LX (100MB/s memcpy)
> 
> 					eager 	get
> min. Latency µs		8.62		9.98
> max Bandwidth MB/s	48.03	100.02
> half bandwith point KB	0.3		2.5
> 
> You can easily see that the eager has better latency for very short messages 
> and that the get has a max bandwidth beeing equivalent of a memcpy (single 
> copy).
> 
> True zero copy has unlimited (sigh!) bandwidth within an SMP and does not 
> really make sense in contrast to a network.

if you can avoid to enter kernel, you'd better do that, because entering
kernel will take much more time than the copy itself.

with the shm/futex approch you can also have a ring buffer to handle
parallelism better while it's at the same time zerocopy and enterely
userspace based in the best case (thought that's not the common case).

thanks,

Andrea

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