OCTOBER 2020 PCWorld 27launching later this year.
HOW MICROSOFT
DIRECTSTORAGE AND
RTX IO WORK
“Games have pushed PC IO and file systems
to the breaking point,” Huang said.
DirectStorage was built to smash past that.
Traditionally, CPUs have both called game
assets from your storage and decompressed
them, passing the data through the system
memory over to your graphics card.
Microsoft’s Andrew Yeung explained why that
worked well before, but not in an era of
blazing-fast PCIe 4.0 NVMe drives:
“Previous gen games had an asset
streaming budget on the order of 50MB/s
which even at smaller 64k block sizes (ie. one
texture tile) amounts to only hundreds of IO
requests per second. With multi-gigabyte a
second capable NVMe drives, to take
advantage of the full bandwidth, this quickly
explodes to tens of thousands of IO requests a
second. Taking the Series X’s 2.4GB/s capable
drive and the same 64k block sizes as an
example, that amounts to >35,000 IO
requests per second to saturate it.
Existing APIs require the [game] to manage
and handle each of these requests one at a
time first by submitting the request, waiting for
it to complete, and then handling its
completion. The overhead of each request is
not very large and wasn’t a choke point for
older games running on slower hard drives,
but multiplied tens of thousands of times per
second, IO overhead can quickly become too
expensive preventing games from being able
to take advantage of the increased NVMe
drive bandwidths.”
In today’s world of 100GB-plus games
with massive file textures and ludicrously fast
PCIe 4.0 SSDs, that traditional CPU handoff
has become the bottleneck.
But while CPU threads need to complete
a task before moving onto the next one, GPUs
excel at executing many tasks in parallel.
DirectStorage takes advantage of that by
letting ultra-fast NVMe SSDs send data
directly to the ultra-fast dedicated VRAM onThe Xbox Series X uses Microsoft’s DirectStorage.