What Makes Server Applications Different?

The large caches and high integer core (cluster) count in one Orochi die (four CMT module Bulldozer die) made quite a few people suspect that the Bulldozer design first and foremost was created to excel in server workloads. Reviews like our own AMD FX-8150 launch article have revealed that single-threaded performance has (slightly) regressed compared to the previous AMD CPUs (Istanbul core), while the chip performs better in heavy multi-threaded benchmarks. However, high performance in multi-threaded workstation and desktop applications does not automatically mean that the architecture is server centric.

A more in depth analysis of the Bulldozer architecture and its performance will be presented in a later article as it is out of the scope of this one. However, many of our readers are either hardcore hardware enthusiasts or IT professionals that really love to delve a bit deeper than just benchmarks showing if something is faster/slower than the competition, so it's good to start with an explanation of what makes an architecture better suited for server applications. Is the Bulldozer architecture a “server centric architecture”?

What makes a server application different anyway?

There have been extensive performance characterizations on the SPEC CPU benchmark, which contains real-world HPC (High Performance Computing), workstation, and desktop applications. The studies of commercial web and database workloads on top of real CPUs are less abundant, but we dug up quite a bit of interesting info. In summary we can say that server workloads distinguish themselves from the workstation and desktop ones in the following ways.

They spend a lot more time in the kernel. Accessing the network stack, the disk subsystem, handling the user connections, syncing high amounts of threads, demanding more memory pages for expending caches--server workloads make the OS sweat. Server applications spend about 20 to 60% of their execution time in the kernel or hypervisor, while in contrast most desktop applications rarely exceed 5% kernel time. Kernel code tends to be very low IPC  (Instructions Per Clockcycle) with lots of dependencies.

That is why for example SPECjbb, which does not perform any networking and disk access, is a decent CPU benchmark but a pretty bad server benchmark. An interesting fact is that SPECJBB, thanks to the lack of I/O subsystem interaction, typically has an IPC of 0.5-0.9, which is almost twice as high as other server workloads (0.3-0.6), even if those server workloads are not bottlenecked by the storage subsystem.

Another aspect of server applications is that they are prone to more instruction cache misses. Server workloads are more complex than most processing intensive applications. Processing intensive applications like encoders are written in C++ using a few libraries. Server workloads are developed on top of frameworks like .Net and make of lots of DLLs--or in Linux terms, they have more dependencies. Not only is the "most used" instruction footprint a lot larger, dynamically compiled software (such as .Net and Java) tends to make code that is more scattered in the memory space. As a result, server apps have much more L1 instruction cache misses than desktop applications, where instruction cache misses are much lower than data cache misses.

Similar to the above, server apps also have more L2 cache misses. Modern desktop/workstation applications miss the L1 data cache frequently and need the L2 cache too, as their datasets are much larger than the L1 data cache. But once there, few applications have significant L2 cache misses. Most server applications have higher L2 cache misses as they tend to come with even larger memory footprints and huge datasets.

The larger memory footprint and shrinking and expanding caches can cause more TLB misses too. Especially virtualized workloads need large and fast TLBs as they switch between contexts much more often.

As most server applications are easier to multi-thread (for example, a thread for each connection) but are likely to work on the same data (e.g. a relational database), keeping the caches coherent tends to produce much more coherency traffic, and locks are much more frequent.

Some desktop workloads such as compiling and games have much higher branch misprediction ratios than server applications. Server applications tend to be no more branch intensive than your average integer applications.

Quick Summary

The end result is that most server applications have low IPC. Quite a few workstation applications achieve 1.0-2.0 IPC, while many server applications execute 3 to 5 times fewer instructions on average per cycle. Performance is dominated by Memory Level Parallelism (MLP), coherency traffic, and branch prediction in that order, and to a lesser degree integer processing power.

So is "Bulldozer" a server centric architecture? We'll need a more in-depth analysis to answer this question properly, but from a high level perspective, yes, it does appear that way. Getting 16 threads and 32MB of cache inside a 115W TDP power consumption envelope is no easy feat. But let the hardware and benchmarks now speak.

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  • mino - Wednesday, November 16, 2011 - link

    IT had most likely to do with you running it on NetBurst (judging by no VT-X moniker).

    As much to do with VT-X as with a crappy CPU ... wiht bus architecture ah, thank god they are dead.
  • JustTheFacts - Wednesday, November 16, 2011 - link

    Please explain why there is no comparison between the latest AMD processors to Intel's flagship two-way server processors: the Intel Westmere-EX e7-28xx processor family?

    Lest you forgot about them, you can find your own benchmarks of this flagship Intel processor here: http://www.anandtech.com/show/4285/westmereex-inte...

    Take the gloves off and compare flagship against flagship please, and then scale the results to reflect the price differece if you have to, but there's no good reason not to compare them that I can see. Thanks.
  • duploxxx - Thursday, November 17, 2011 - link

    Westmere EX 2sockets is dead, will be killed by own intel platform called romley which will have 2p and 4p.

    it was a stupid platform from the start and overrated by sales/consultants with there so called huge memory support.
  • aka_Warlock - Wednesday, November 16, 2011 - link

    I think you should have done a more thorough VM test than you did. 64GB RAM?
    We all know single threaded performance is weak, but I still feel the server are underutilized in your test.

    These CPU's are screaming heavy multi threading workloads. Many VM's. Many vCPU's.

    What would the performance be if you had, say, at least 192GB of RAM and 50 (maybe more) VM's on it?

    And offcourse, storage should not be a bottleneck.

    I think this is where his 8modules/16threads cpu would shine.
    A dual socket rack/blade. 16modules/32 threads.
    Loads of RAM and a bounch of VM's.
  • iwod - Wednesday, November 16, 2011 - link

    It is power hungry, isn't any better then Intel, and it is only slightly cheaper, at the cost of higher electricity bill.

    So unless with some software optimization that magically show AMD is good at something, i think they are pretty much doomed.

    It is like Pentium 4, except Intel can afford making one or two mistakes, but not with AMD.
  • mino - Wednesday, November 16, 2011 - link

    Then the article served its purpose well.
  • SunLord - Wednesday, November 16, 2011 - link

    So is the AMD system running 8GB DDR3-1600 DIMMS or 4GB DDR3-1333? Because you list the same DDR3-1333 model for both systems and if the Server supports 16 DIMMs well 16*4 is 64GB
  • JohanAnandtech - Thursday, November 17, 2011 - link

    Copy and paste error, Fixed. We used DDR-3 1600 (Samsung)
  • Johnmcl7 - Wednesday, November 16, 2011 - link

    I have wondered about this, with more cores per socket and virtualisation (organising new set of servers and buying far less hardware for the same functionality) so I'd have thought in total less server hardware is being purchased. Clearly that isn't the case though, is the money made back from more expensive servers?

  • bruce24 - Wednesday, November 16, 2011 - link

    While sure which each new generation of server you need much less hardware to do the same amount of work, however worldwide people are looking for servers to do much more work. Each year companies like Google, Facebook, Amazon, Microsoft and Apple add much more computing power than they could get by refreshing their current servers.

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