Re: [PATCH v3] Weighted interleave auto-tuning

[Gregory Price <gourry@gourry.net>]

On Fri, Jan 24, 2025 at 05:58:09AM +0000, Matthew Wilcox wrote:
> On Wed, Jan 15, 2025 at 10:58:54AM -0800, Joshua Hahn wrote:
> > On machines with multiple memory nodes, interleaving page allocations
> > across nodes allows for better utilization of each node's bandwidth.
> > Previous work by Gregory Price [1] introduced weighted interleave, which
> > allowed for pages to be allocated across NUMA nodes according to
> > user-set ratios.
> 
> I still don't get it.  You always want memory to be on the local node or
> the fabric gets horribly congested and slows you right down.  But you're
> not really talking about NUMA, are you?  You're talking about CXL.
> 
> And CXL is terrible for bandwidth.  I just ran the numbers.
> 
> On a current Intel top-end CPU, we're looking at 8x DDR5-4800 DIMMs,
> each with a bandwidth of 38.4GB/s for a total of 300GB/s.
> 
> For each CXL lane, you take a lane of PCIe gen5 away.  So that's
> notionally 32Gbit/s, or 4GB/s per lane.  But CXL is crap, and you'll be
> lucky to get 3 cachelines per 256 byte packet, dropping you down to 3GB/s.
> You're not going to use all 80 lanes for CXL (presumably these CPUs are
> going to want to do I/O somehow), so maybe allocate 20 of them to CXL.
> That's 60GB/s, or a 20% improvement in bandwidth.  On top of that,
> it's slow, with a minimum of 10ns latency penalty just from the CXL
> encode/decode penalty.
>

From the original - the performance tests show considerable opportunity
in the scenarios where DRAM bandwidth is pressured - as you can either

1) Lower the DRAM bandwidth pressure by offloading some cachelines to
   CXL - reducing latency on DRAM and reducing average latency overall.

   The latency cost on CXL lines gets amortized over all DRAM fetches
   no longer hitting stalls.

2) Under full-pressure scenarios (DRAM and CXL are saturated), the
   additional lanes / buffers provide more concurrent fetches - i.e.
   you're just doing more work (and avoiding going to storage).

   This is the weaker of the two scenarios.

No one is proposing we switch the default policy to weighted interleave.

= Performance summary =
(tests may have different configurations, see extended info below)
1) MLC (W2) : +38% over DRAM. +264% over default interleave.
   MLC (W5) : +40% over DRAM. +226% over default interleave.
2) Stream   : -6% to +4% over DRAM, +430% over default interleave.
3) XSBench  : +19% over DRAM. +47% over default interleave.

=====================================================================
Performance tests - MLC
From - Ravi Jonnalagadda <ravis.opensrc@micron.com>

Hardware: Single-socket, multiple CXL memory expanders.

Workload:                               W2
Data Signature:                         2:1 read:write
DRAM only bandwidth (GBps):             298.8
DRAM + CXL (default interleave) (GBps): 113.04
DRAM + CXL (weighted interleave)(GBps): 412.5
Gain over DRAM only:                    1.38x
Gain over default interleave:           2.64x

Workload:                               W5
Data Signature:                         1:1 read:write
DRAM only bandwidth (GBps):             273.2
DRAM + CXL (default interleave) (GBps): 117.23
DRAM + CXL (weighted interleave)(GBps): 382.7
Gain over DRAM only:                    1.4x
Gain over default interleave:           2.26x

=====================================================================
Performance test - Stream
From - Gregory Price <gregory.price@memverge.com>

Hardware: Single socket, single CXL expander

Summary: 64 threads, ~18GB workload, 3GB per array, executed 100 times
Default interleave : -78% (slower than DRAM)
Global weighting   : -6% to +4% (workload dependant)
mbind  weights     : +2.5% to +4% (consistently better than DRAM)

=====================================================================
Performance tests - XSBench
From - Hyeongtak Ji <hyeongtak.ji@sk.com>

Hardware: Single socket, Single CXL memory Expander

NUMA node 0: 56 logical cores, 128 GB memory
NUMA node 2: 96 GB CXL memory
Threads:     56
Lookups:     170,000,000

Summary: +19% over DRAM. +47% over default interleave.


> Putting page cache in the CXL seems like nonsense to me.  I can see it
> making sense to swap to CXL, or allocating anonymous memory for tasks
> with low priority on it.  But I just can't see the point of putting
> pagecache on CXL.

No one said anything about page cache - but it depends.

If you can keep your entire working set in-memory and on-CXL, as opposed
to swapping to disk - you win.  "Swapping to CXL" incurs a bunch of page
faults, that sounds like a lose.

However - the stream test from the original proposal agrees with you
that just making everything interleaved (code, pagecache, etc) is at
best a wash:

Global weighting   : -6% to +4% (workload dependant)

But targeting specific regions can provide a modest bump

mbind weights      : +2.5% to +4% (consistently better than DRAM)

~Gregory