IPC Increases: Double L1 Data Cache, Better Branch Prediction

One of the biggest changes in the design is the increase in the L1 data cache, doubling its size from 64 KB to 128 KB while keeping the same efficiency. This is combined with a better prefetch pipeline and branch prediction to reduce the level of cache misses in the design. The L1 data cache is also now an 8-way associative design, but with the better branch prediction when needed it will only activate the one segment required and when possible power down the rest.  This includes removing extra data from 64-bit word constructions. This reduces power consumption by up to 2x, along with better clock gating and minor adjustments. It is worth pointing out that doubling the L1 cache is not always easy – it needs to be close to the branch predictors and prefetch buffers in order to be effective, but it also requires space. By using the high density libraries this was achieved, as well as prioritizing lower level cache. Another element is the latency, which normally has to be increased when a cache increases in size, although AMD did not elaborate into how this was performed.

As listed above, the branch prediction benefits come about through a 50% increase in the BTB size. This allows the buffer to store more historic records of previous interactions, increasing the likelihood of a prefetch if similar work is in motion. If this requires floating point data, the FP port can initiate a quicker flush required to loop data back into the next command. Support for new instructions is not new, though AVX2 is something a number of high end software packages will be interested in using in the future.

These changes, according to AMD, relate to a 4-15% higher IPC for Excavator in Carrizo compared to Steamroller in Kaveri.  This is perhaps a little more what we normally would expect from a generational increase (4-8% is more normal), but AMD likes to stress that this comes in addition to lower power consumption and with a reduced die area. As a result, at the same power Carrizo can have both an IPC advantage and a frequency advantage.

As a result, AMD states that for the same power, Cinebench single threaded results will go up 40% and multithreaded results up 55%. The benefits are fewer however the further up the power band you go despite the increase, as the higher density libraries perform slightly worse at higher power than Kaveri.

Efficiency and Die Area Savings Power Saving and Power Consumption
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  • yankeeDDL - Wednesday, June 3, 2015 - link

    If you think as gaming as hard-core gaming, latest and greatest titles, max settings, high res, then yes, I agree.
    But, as a father, I can tell you that there are tons of kids that are OK with "some" gaming and for whom the parents won't fork 1Kusd for a powerful gaming rig, and for which AMD really offers a price/performance ratio that cannot be compared.
    Reply
  • FlushedBubblyJock - Tuesday, June 9, 2015 - link

    having kids in usa is down down down, so not so much biz there Reply
  • barleyguy - Thursday, June 4, 2015 - link

    The marketing for this particular launch is specifically targeting players of MOBAs, such as League of Legends, DOTA2, and Starcraft. The active players of those 3 games is about 100 million people. You can argue that's not a majority of users, but AMD has never had a majority share of laptops, and 100 million people is a sizable market.

    I expect that AMD will make more direct appeals for that group of people, such as sponsoring tournaments, and generally trying to establish mindshare for laptops in that group.

    I don't think targeting that particular niche market is a bad idea, personally.
    Reply
  • Refuge - Thursday, June 4, 2015 - link

    Their APU's can perform on those games very well, but also so can all of Intel's HD series graphics, at least every rendition I've ever seen.

    It won't be an easy crowd to win over. Even if the requirements for their sport only warrant a $300 computer, you know they are gamers and still want their supercomputers.
    Reply
  • Magichands8 - Wednesday, June 3, 2015 - link

    I agree with this. But it also doesn't make sense to me why they have. CPU's have been commodity devices for some time now and it makes no sense to me whatsoever that the average consumer would buy Intel when they can transparently get the very same performance for their needs with a much cheaper AMD processor. This, plus the fact that AMD does so well with essentially embedded systems (XBox, Playstation) even though they are 'AMD Inside' suggests to me that their struggle is largely due to marketing failures rather than technological ones. Reply
  • medi03 - Thursday, June 4, 2015 - link

    Athlon 64th were outsold by Intel's Prescott that was:
    a) slower
    b) consumed more power
    c) more expensive

    also because of "AMD's marketing failures" I guess...
    Reply
  • FlushedBubblyJock - Tuesday, June 9, 2015 - link

    no it's the skin that surrounds the amd heart - and that skin is cheap and ugly - that's how amd plays their cards... a low class dirt bucket cheap compile - so even if it could be made to look great and "feel very expensive and well constructed" - those creating AMD skin WON'T DO IT. Reply
  • FlushedBubblyJock - Tuesday, June 9, 2015 - link

    Thinking again I will admit when browsing new laptops in person AMD GAMING ! is not something I can ever recall seeing ... it's very tough sometimes just to get the basic specs visible by the box stores.... Reply
  • AS118 - Wednesday, June 3, 2015 - link

    I don't ind 1366 x 768 on a smaller screen (14" and less for me) and think that AMD's performance on 900p and below's great. I also think that it's sad that more people don't know about them and how capable they are.

    Many casual customers who would want that extra gaming performance don't even know it exists on A8's and A10's.
    Reply
  • AS118 - Saturday, June 6, 2015 - link

    Yeah, I don't mind 1366 x 768 myself on smaller laptops, and gaming at that resolution is a thing with AMD APU's. Gaming performance on the A8 and especially A10 APU's is why I've been getting AMD Processors on Laptops for years.

    I don't need a lot of GPU oomph on my laptops, just needed the option to play casual games and not that hard to run stuff like WoW (well, before WoD at least) on the go.
    Reply

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