Out of Order Loads done right

Since the Pentium Pro, x86 CPUs have been capable of issuing and executing instructions out of order. However, on average one third of the instructions in those reorder buffers could not be reordered easily: we are talking about loads. Moving loads forward can give a very big boost to performance. Instead of loading a piece of data when you need it, it is much more useful to start the load as early as you can. That way, L1 and even L2-cache latencies can be much more easily hidden.

This is pretty easy to understand. Imagine having an ALU operation that needs a certain piece of data but that the data is not available in the L1-cache. If the load has been executed many cycles before the ALU operation needs that piece of data, the L2-cache latency is going to have a reduced impact. Of course, you don't want to load a value which is being or will be written to by a previous - following the program/thread order - store. That would mean you are loading an old value, not the up to date one. Check out the picture below.


Load 2 cannot be moved forward, since it has to wait until the first Store is done. Only after Store 1 is done will variable Y have its correct value. However there is no reason why Load 4 cannot move forward. It doesn't have to wait for Store 3 and store 1 to finish. By moving Load 4 forward, you give the load unit more time to get the right operand, as we assume that after load 4 a calculation with operand Y will happen.

Currently, CPUs will generally delay load 4 when a store is in flight (active). The problem is the address to which the stores will write has yet to be calculated. To be more precise, the memory addresses are still unknown during reordering and scheduling. When a Load micro-op enters the ROB, the memory addresses of previous stores (from the program order) are not known until they pass the AGU (Address Generation Units).

However, the risk that a load will load a value out of an address that is being written to by a store that has yet to be finished is pretty small (1-2%). That is why Jack Doweck of the Core development team decided to allow Loads to go ahead of previous stores, assuming that the load will not be loading information that will be updated by that preceding store. To avoid that the assumption was wrong, a predictor is used to help. The dynamic alias predictor tries to predict whether or not a previous store will write to the same address as the address from which the load - that you want to execute earlier, thus out of order - will load its data.

Based on Jack Doweck's comments and a study of Intel's previous P6 and P-M architectures I drew up the scheme below. Be warned that this is not the official Intel diagram.


The predictor gives the ReOrder Buffer (ROB) the permission to move a load ahead of a store or not. After the Load has been moved ahead and executed, the conflict logic scans the store buffer located in the Memory reOrder Buffer (MOB) to see if any of the stores which were located before the load (following program order) have written to the address of the out of order load. If so, the load must be redone, and the misprediction penalty is about 20 lost cycles. (Note that the branch misprediction penalty is also about 20 cycles). Worst case, the new dynamic alias predictor may slightly reduce performance, but realistically it's four steps forward, one step back, resulting in a net performance boost.

Determining whether a load and a store share the same address is called memory disambiguation. Allowing loads to move ahead of stores gives a big performance boost. In some snippets of benchmarking code, Intel saw up to a 40% performance boost, solely the result of the more flexible way Loads get reordered. It is pretty clear that we won't see this in most real applications, but it is nevertheless impressive and it should show tangible (10-20%) performance boosts together with the fast L2 and L1 cache.

Let us not forget that loads are probably the most important instructions of all. Not only are loads about one third of the micro-ops that are in flight in a x86 CPU, but they can also cause costly stalls when a load needs to go to the L2 cache (or worse, system memory). So how does this super flexible reordering of loads compare with other architectures?


The P6 and P-M could already reorder Loads pretty good. They could move one Load before other Loads, as well as before Stores which have no unknown addresses or addresses which do not reference the same address as the load. In contrast, the Athlon 64 can only move loads before independent ALU operations (ADD etc.). Loads cannot be moved ahead much at all to minimize the effect of a cache miss, and other loads cannot be used to keep the CPU busy if a load has to wait for a store to finish. This means that the Athlon 64 processor is severely limited when it comes to reorder code.

This is probably one of the most important reasons why the Athlon 64 does not outperform the P-M in gaming and integer workloads despite having a lower latency memory system and more integer execution sources. Integer workloads tend to jump around in memory, and have many unknown addresses which must be calculated first. It is less important for FP intensive loads, which is also one of the reasons why the Athlon 64 had no problem with Dothan in this kind of workload. FP workloads access the memory in a much more regular fashion.

Once Loads and Stores are in the queues of Load/Store units, the Athlon's L/S unit allows Loads to bypass Stores, except of course when the load would bypass a store to the same address. Unfortunately, by then the Loads are already out of the ICU and cannot be used to fill the holes that dependencies and cache misses make. You could say that the Athlon (64) has some Load/Store reordering but it's much later in the pipeline and is less flexible than the P6, P-M, and Core architectures.

Out of Order Execution Concluding Thoughts
POST A COMMENT

85 Comments

View All Comments

  • Betwon - Tuesday, May 02, 2006 - link

    P6: sub [mem],eax decodes to three micro-ops
    Core duo: sub [mem],eax decodes to two micro-ops
    K8: sub [mem],eax decodes to one macro-op

    P6: sub eax,[mem] decodes to two micro-ops
    Core duo: sub eax,[mem] decodes to one micro-op
    K8: sub eax,[mem] decodes to one macro-op

    Intel's micro-fusion is different with the K7/K8's macro-op.

    P4 has 2X2 int ALU and 2 AGU.
    K7/K8 has 3 int ALUand 3 AGU.
    But Core duo has only 2 int ALU and 2 AGU.

    The integer performance:
    Core duo>K7/K8

    Why?
    Because Core duo's length of the depenency chain of the critical path is the shorest.
    The most integer program asm codes can be thought as a high and thin tree of the depenency chain, (the longest depenency chain is called the critical path)
    The critical path determines the performance. The length of the depenency chain of the critical path is the cycles needed to complete this critical path.
    Core duo(2ALU/2AGU) spends less cycles than K7/K8(3ALU/3AGU) -- Because more INT funtions can not accelerate the true dependency atoms-operations.

    The P-M/Core duo's special ability of anti true dependency atoms-operations is the real reason of it's INT outperformance, which is different with old P6(such as Pentium 3).

    The most FP program asm codes can be thought as a boskage (there are many short depenency chains). The best ILP can be performed--The more FP FADD/FMUL funtions, the more performence.

    Double the FP funtions or double the speed of half-speed FP functions is a good idea for the most FP programs.
    But double INT funtions do not always enhence the INT preformence so greatly.
    Conroe only has three ALUs and two AGUs, but not 4 ALU and 4AGU.
    K7/K8 has three ALUs and three AGUs.
    Reply
  • Betwon - Tuesday, May 02, 2006 - link

    Sorry, I'm not one from a English-language nations. I just tell my idea with many spelling or syntax errors.
    funtions -- funtion units

    I want to tell why Conroe with only 3ALU/2AGU has the INT outperformance. Core has the superexcellent ability to process the true dependency chains.(much much better than K8 3ALU/3AGU).
    Even Core duo with 2ALU/2AGU has the INT outperformance(much better than K8 3ALU/3AGU).

    The length of the depenency chain of the critical path
    The true dependency atom-operations
    Reply
  • Starglider - Monday, May 01, 2006 - link

    A truly excellent article. I just have a couple of questions;

    quote:

    The second and more important advantage is the on die memory controller, which lowers the latency to the memory considerably. However, the lower clockspeeds of the Core CPUs (relative to NetBurst) and the faster FSB also lower latency significantly. With the numbers available to us now, we have reason to believe that the Athlon 64 X2's latency advantage will shrink to only 15 to 20%.


    It's clear that increasing FSB speed can reduce memory latency. However I'm not clear why lower CPU core speed will reduce absolute latency - sure it will reduce the number of CPU cycles that occur while waiting for memory, but how can it reduce the absolute delay?

    You don't seem to have included inter-instruction latency in your comparison tables. I know this data can be hard to get hold of, but it's critical to the performance of highly serial code (e.g. the pi calculation benchmarks that seem to be so popular at the moment). Is there any chance it could be included?

    Finally I'm wondering if Intel will revist some of the P4 clock speed enhancing tricks later on. Things like LVS and double pumped AUs would only have slowed down an already complex development process that Intel desperately needed to be completed quickly. But if AMD do come out with a new architecture that matches or exceeds Conroe on IPC, Intel might be able to respond quite quickly by bringing back some of their already well understood clock speed tricks to accelerate Conroe.
    Reply
  • Makaveli - Monday, May 01, 2006 - link

    You need to get away from thinking of increased clockspeed for extra performance, the future is multicore Cpu's and parellism. Reply
  • Starglider - Tuesday, May 02, 2006 - link

    I write heavily multithreaded applications for a living, but sometimes there is just no substitute for fast serial execution; a lot of things just can't be parallelised. Serial execution speed is effectively IPC * clock rate, so yes increasing clock rate is still very helpful as long as IPC doesn't suffer. Reply
  • saratoga - Monday, May 01, 2006 - link

    ^^ Did you read the post you replied to? His point is valid.

    Lower clock speed is not going to improve memory latency. It may mean that latency is less painful, but if you took two core chips, one at 2GHz and the other at 3GHz, the absolute latency is roughly if not exactly the same for each. Though the cost of each ns of latency is 50% more dear on the 3GHz chip.
    Reply
  • Spoonbender - Tuesday, May 02, 2006 - link

    It would be more accurate to say that each cycle of latency is more dear on the 2ghz chip, wouldn't it? ;)
    What matters is how *long* the latency is, in ns. A ns latency is a ns, and it forces the cpu to wait exactly one ns, no matter its clock speed... :)

    So yeah, definitely a valid point, and I wondered about that in the article as well.
    Reply
  • saratoga - Wednesday, May 03, 2006 - link

    quote:

    It would be more accurate to say that each cycle of latency is more dear on the 2ghz chip, wouldn't it? ;)


    No. Its 50% worse for the 3GHz chip (since the clock speed is 50% higer).

    quote:

    What matters is how *long* the latency is, in ns. A ns latency is a ns, and it forces the cpu to wait exactly one ns, no matter its clock speed... :)


    And one ns is how many clock cycles on a 2GHz chip? And how many of a 3GHz chip? Think this through . . .
    Reply
  • Spoonbender - Wednesday, May 03, 2006 - link

    quote:

    No. Its 50% worse for the 3GHz chip (since the clock speed is 50% higer).

    No it isn't. They both waste *exactly* one ns of execution per, well, ns of latency. How many cycles they can cram into that ns is irrelevant.

    [quote]
    And one ns is how many clock cycles on a 2GHz chip? And how many of a 3GHz chip? Think this through . .
    [/quote]
    Yeah, of course, with 1 ns latency, a 3ghz chip will waste more clock cycles than a 2ghz chip, yes. That's obvious. But they will both lose exactly 1 ns worth of execution. That's what matters. Not the number of clock cycles.
    If they both perform equally well, despite the clock speed difference, then adding 1 ns latency to both will have exactly the same impact on both. Yes, the 3ghz chip will lose more clock cycles, but the 2ghz will (if we stick with the assumption of similar performance), have a higher IPC, and so waste the same amount of actual work.

    If you like, look at Athlon 64 and P4.
    If both cpu's waste one clock cycle, then the A64 takes the biggest impact, because of its higher IPC.
    If both cpu's waste one ns, it doesn't change anything. True, the P4 loses the biggest amount of cycles, but as I said before, the A64 loses more *per* cycle. The net result is that they both lose, wait for it, *one* nanosecond's worth of execution.
    Reply
  • BigT383 - Monday, May 01, 2006 - link

    I loved this article. It's due to articles like these that I've been reading Anandtech since before the days of the K6-2. Reply

Log in

Don't have an account? Sign up now