Re: [patch 3/4] net: Percpufy frequently used variables -- proto.sockets_allocated

[Eric Dumazet <dada1@cosmosbay.com>]

Andrew Morton a écrit :
> Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
> > On Fri, Jan 27, 2006 at 09:53:53AM +0100, Eric Dumazet wrote:
> > > Ravikiran G Thirumalai a écrit :
> > > > Change the atomic_t sockets_allocated member of struct proto to a 
> > > > per-cpu counter.
> > > >
> > > > Signed-off-by: Pravin B. Shelar <pravins@calsoftinc.com>
> > > > Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
> > > > Signed-off-by: Shai Fultheim <shai@scalex86.org>
> > > Hi Ravikiran
> > >
> > > If I correctly read this patch, I think there is a scalability problem.
> > >
> > > On a big SMP machine, read_sockets_allocated() is going to be a real killer.
> > >
> > > Say we have 128 Opterons CPUS in a box.
> > read_sockets_allocated is being invoked when when /proc/net/protocols is read,
> > which can be assumed as not frequent.  
> > At sk_stream_mem_schedule(), read_sockets_allocated() is invoked only 
> > certain conditions, under memory pressure -- on a large CPU count machine, 
> > you'd have large memory, and I don't think read_sockets_allocated would get 
> > called often.  It did not atleast on our 8cpu/16G box.  So this should be OK 
> > I think.
>
> That being said, the percpu_counters aren't a terribly successful concept
> and probably do need a revisit due to the high inaccuracy at high CPU
> counts.  It might be better to do some generic version of vm_acct_memory()
> instead.

There are several issues here :

alloc_percpu() current implementation is a a waste of ram. (because it uses 
slab allocations that have a minimum size of 32 bytes)

Currently we cannot use per_cpu(&some_object, cpu), so a generic version of 
vm_acct_memory() would need a rework of percpu.h and maybe this is not 
possible on every platform ?

#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))

-->

#define per_cpu_name(var) per_cpu__##var
#define per_cpu_addr(var) &per_cpu_name(var)
#define per_cpu(var, cpu) (*RELOC_HIDE(per_cpu_addr(var), __per_cpu_offset[cpu])


But this could render TLS migration difficult...

Eric
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