maximumpc.com aug 2019 MAXIMUMPC 15
Jarred Walton
Tech Talk
Jarred Walton has been a
PC and gaming enthusiast
for over 30 years.AMD Reveals Navi and
Its RDNA Architecture
Perhaps the biggest change from GCN is that AMD
has reworked the way instructions are grouped.
In GCN, AMD used a Wave64 work-item that
combined work for 64 GPU threads. Each Wave
would end up being split into four chunks of work
that would be sent to a SIMD16 structure, requiring
four cycles to execute the work-item. If the Wave
didn’t fully utilize 64 threads, no matter: It still took
four cycles. A shared scalar unit would handle
interleaving the workload on to the SIMD unit.
For RDNA, AMD GPUs now have a base Wave
work-item, and it feeds into a SIMD32 vector unit.
There are also two scalar units for scheduling the
workloads, which increases throughput to one
Wave32 per cycle. Not only do heavy workloads
end up executing faster, but lighter workloads—
where only 16 threads of work are needed, for
example—now complete in one cycle instead of
four. Basically, everything becomes more efficient.
AMD also adds a new L1 cache to the GPU
caching hierarchy, joining the existing L0 and
L2 caches, and RDNA features dual compute
units with some shared resources to keep things
plugging along—AMD calls this a Workgroup
Processor. Wider data paths connect the pieces
together, and the full graphics pipeline can now
work directly with compressed color data.
The net benefit is that RDNA should deliver
roughly 25 percent better performance per clock—
or IPC (instructions per clock). And while I haven’t
been able to run benchmarks yet, AMD claims the
9.75 Tflops RX 5700 XT should perform slightly
better than the 12.67 Tflops RX Vega 64. It also
manages this with 40 CUs compared to Vega’s 64
CUs. But it’s not just about raw performance. TheAMD’s l Ast MAjor revAMp of its GPU architecture was in 2012, with the
HD 7970 and related first-gen GCN graphics cards. Everything since then,
up through the Vega 20 GPU at the heart of the Radeon VII, has used a
variation of GCN. At E3, and with the official unveiling of the Radeon RX
5700 XT, AMD finally gave us the skinny on its plans for the next generation
of GPUs, code-named Navi, and using a new RDNA architecture.
RX 5700 XT will also have a TBP
(typical board power) of 225W,
compared to Vega 64’s 295W.
That’s an excellent improvement
if true, and AMD says performance
per watt is 50 percent better than
the last-gen GCN architecture. It
needs to be a major improvement,
as Nvidia has been killing AMD on
GPU efficiency for seven years.
What’s alarming is that RDNA
appears to continue a lot of the
GCN legacy. Navi is manufactured
at 7nm, yet performance and
efficiency look to be at the same
level as Nvidia’s three-year-
old 16nm Pascal architecture.
The RX 5700 should equal the
performance of Nvidia’s RTX 2070,
but without any ray tracing or deeplearning enhancements. (Ray
tracing will come in next year’s
Navi 20 and RDNA 2.) It looks like
Pascal rather than Turing. And the
12nm RTX 2070 is a 175W TBP part,
while the 5700 XT is 225W TBP.
RDNA and Navi are moving in
the right direction, but Nvidia has
a response—it will be launching
the RTX 2060 Super and RTX 2070
Super (and later the 2080 Super).
Performance should be up to 25
percent faster than existing cards,
so the target AMD was aiming for
has moved. I’ll look at new GPUs
from both companies next month.Navi makes some sweeping changes, but will they be enough?©^
AMD