In your quest for the highest frame rates, the prettiest textures, and the
most karma in the PC master race
subreddit, you decided to go with a multi-GPU setup for your new gaming rig.
But when you fire up your games, you notice that the sick gains you're
getting aren't really what you expected, especially after
dropping so much money on your system.
So, what's the deal with that? Well, this problem has plagued PC gamers for
over a decade. Why don't you usually get
double the performance from a dual card configuration or triple if you have
three video cards? I mean, since graphics rendering benefits from having
more computing units in parallel, which you can learn more about in this video,
by the way, wouldn't it make sense then for multicard setups like SLI and
Crossfire to scale pretty easily?
Unfortunately, it's not quite that simple. For starters, using two separate
graphics cards means they have to talk to each other somehow. You see, most
modern multiGPU setups use something called alternate frame rendering, where
each card renders every other frame
separately. Sounds efficient, but for
them to be pushed to your monitor in the right order, the cards have to sync up
properly, creating a surprising amount of overhead for both the GPUs and the
electrical communication paths or buses that link them. This overhead not only
gets worse as you add more cards to your rig, but can even result in a very
unpleasant phenomenon called micro stuttering, which often shows up even if
you're getting lots of frames per second. You see, for smooth gameplay,
you need not only high FPS, but good and
consistent frame times, which is the
amount of time that a given frame will remain on your screen. Poor
synchronization between the cards can leave certain frames on your monitor for
way longer than they should be, even if the total number of frames each second
is very high. This can render certain games
unplayable, even though your setup is more than powerful enough to run it on
paper. And as if that wasn't enough, an
even bigger problem is that it's very difficult for your GPUs to know what
frame to render next since it depends heavily on what the player chooses to
do. The real time nature of game rendering makes it tough to parallelize
in certain games among multiple GPUs,
even with alternate frame rendering, as there isn't always a second frame or
even part of a predicted one for your second card to work on. Of course, it
isn't all about the inherent limitations of your hardware. On the software side,
some game developers have been able to code their games in a manner that makes
it easier for multiple GPUs to split the workload, while others simply don't
bother. With the difficulty in coding for a very small subset of customers,
many devs don't see the point in sinking
tons of time into making sure that their games will play nice with multicard
systems. Some games even manage to have worse performance in these situations.
And finally, you definitely shouldn't run out and buy multiple GPUs if you're
rocking a multiple monitor setup and you want extra video memory or VRAM to store
all of your textures. With SLI and
Crossfire, you are limited only to however much VRAM is on one card because
both GPUs need to be able to access the same information in memory in order to
work properly in tandem, whatever it might say on the packaging. So, does
this mean then that SLI and Crossfire are useless gimmicks and a complete
waste of money? Well, no. Multicard
setups will give you the best possible performance in certain use cases and can
even make some games playable at very high settings and resolutions where they
wouldn't otherwise be, even with the best single card on the market. Not to
mention that multiGPU rendering may improve with new APIs like Vulcan and
DirectX12, which by the way you can learn more about here. Just do your
research before you buy as to what games
see the greatest benefit. And remember to keep your expectations realistic.
After all, you wouldn't expect to be able to drive twice as fast just because
you bought two cars, would you? Unless
you're trying to do like a fan remake of Tokyo
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