Re: [RFC][PATCH 0/9] Network receive deadlock prevention for NBD

From: Peter Zijlstra
Date: Wed Aug 09 2006 - 08:39:20 EST

On Wed, 2006-08-09 at 09:46 +0400, Evgeniy Polyakov wrote:
> On Tue, Aug 08, 2006 at 09:33:25PM +0200, Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> >
> > "Kernel Summit 2005: Convergence of network and storage paths"
> >
> > We believe that an approach very much like today's patch set is
> > necessary for NBD, iSCSI, AoE or the like ever to work reliably.
> > We further believe that a properly working version of at least one of
> > these subsystems is critical to the viability of Linux as a modern
> > storage platform.
> There is another approach for that - do not use slab allocator for
> network dataflow at all. It automatically has all you pros amd if
> implemented correctly can have a lot of additional usefull and
> high-performance features like full zero-copy and total fragmentation
> avoidance.

On your site where you explain the Network Tree Allocator:

You only test the fragmentation scenario with the full scale of sizes.
Fragmentation will look different if you use a limited number of sizes
that share no factors (other than the block size); try 19, 37 and 79
blocks with 1:1:1 ratio.

Also, I have yet to see how you will do full zero-copy receives; full
zero-copy would mean getting the data from driver DMA to user-space
a single copy. The to user-space part almost requires that each packet
on its own page.

As for the VM deadlock avoidance; I see no zero overhead allocation path
you do not want to deadlock your allocator. I see no critical resource
isolation (our SOCK_MEMALLOC). Without these things your allocator might
improve the status quo but it will not aid in avoiding the deadlock we
to tackle here.

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