This question does not assume any specific architecture.
Assume that we have a multicore processor with cache coherence, out-of-order execution, and branch prediction logic. We also assume that stores to memory are strictly in program order.
We have two threads running in parallel, each on a separate core.
Below are the threads’ pseudo-code. data and flag are initially 0.
Thread #1 code:
data=10;
flag=1;
Thread #2 code:
while(!flag);
print data;
With proper synchronization, Thread #2 would eventually print 1. However, the branch predictor could potentially predict that the loop is not entered, thus perform a speculative read of data, which contains 0 at that time (prior to Thread #1 setting data). The prediction is correct, i.e. ‘flag’ is eventually set to 1. In this case the print data instruction can be retired, but it prints the incorrect value of 0.
The question is whether a memory barrier would somehow prevent the speculative read of data, and cause the cpu to execute the busy wait properly. An alternative solution could be to let the branch predictor do its work, but snoop the writes done by the other core, and in case a write to data is detected, we can use the ROB to undo the premature read (and its dependent instructions) and then re-execute with the proper data.
Arch-specific answers are also welcome.
No, branch prediction + speculative execution is fine in an ISA with memory barriers, as long as mis-speculation is killed properly.
thus perform a speculative read of data, which contains 0 at that time
When the CPU detects the misprediction, instructions from the mis-speculated path of execution are discarded, along with their effects on architectural registers.
When the correct path of execution does eventually exit the loop, then the memory barrier will run (again), then the load of data will run (again). The fact that they earlier ran in the shadow of a mis-predicted branch has no effect.
Your pseudo-code assembly isn't very clear because it makes print data look like a single operation. In fact it will involve a load into a register and then a call print instruction.
When the data load runs on the correct path, it will have to redo the work of reading a value from cache, and cache is coherent across cores. It doesn't matter if the mis-speculated load brought the cache line into this core's L1d cache; a store by another core will have to invalidate it before that store can become globally visible.
The loop exits after seeing exit!=0; the barrier after that makes sure that later loads haven't already happened, giving acquire semantics to the load of exit (assuming it includes blocking LoadLoad reordering).
The barrier executing on the correct path makes sure that this core waits for that invalidation instead of using an early load.
A store / release barrier in the writer makes sure that the new data value is globally visible before exit = 1 is visible to any other threads on any cores.
Related
What is the difference between the issue queue and lsq queue for
memory instructions? Do memory instructions pass through both queues, or do they only pass
through the lsq queue.
If they pass through both queues what is their order?
I'm assuming you use the arm-like nomenclature here so the issue queue is what Intel calls RS (reservation station) and by issue you mean sending a uop ready for execution.
The answer is that memory instructions need to pass both. All instructions need to be issued (except the ones that can be eliminated without execution, for example register moves, zero idioms, nops, etc..). Let's rephrase - all instructions that need to go through an ALU need to go through the issue process first. Memory instructions will simply use that step to calculate their addresses.
This is true for loads, for stores there is usually an internal split into store-address and store-data, so the store-address will behave like a load in that sense and calculate its address during that step.
There is usually a dedicated execution port for that and dedicated execution units because the address calculation usually follows one of few specific addressing modes (each architecture has a different set of these), but aside from that the execution needs to follow the same rules like any other operation in the CPU - it needs to have its sources ready and read from the register file or bypassed from an in flight operation, it needs to get arbitrated when the execution port is free and prioritized by the same aging rules, so it makes sense that it uses the common path.
Once the memory operation has finished execution, it will be sent to the LSU (load-store unit, or the DCU, data-cache unit on Intel) and perform the actual memory access using the generated address. The LSU pipe will take care of the address translation, TLB lookups, the page walk if needed (though this is sometimes done in a dedicated unit), the address range and property checks, the cache lookup (if cacheable) and sending a miss to the next cache level or memory if needed. It may also trigger prefetches as part of the process.
For a load, when the LSU pipe has completed (which may require multiple passes and wakeups if the data was not available in the L1), the LSU will signal the issue queue again in order to wakeup anyone who was depended on the result.
For a store, store-address may fetch the line to the cache in advance as an optimization but the actual next step is usually to wakeup after retirement (since stores may not be dispatched to memory while speculative, unless you have some tricks to handle that).
It's also worth to mention that some CPUs try to optimize loads that can forward the data directly from prior stores instead of fetching it from the cache/memory. This can include forwarding (very common) or memory renaming (less common). The former is usually handled by the LSU internally, but the latter can be done much earlier and without the LSU (though the LSU pipe is usually still activated to validate the result).
Alright, so I know that if a particular conditional branch has a condition that takes time to compute (memory access, for instance), the CPU assumes a condition result and speculatively executes along that path. However, what would happen if, along that path, yet another slow conditional branch pops up (assuming, of course, that the first condition hasn't been resolved yet and the CPU can't just commit the changes)? Does the CPU just speculate inside the speculation? What happens if the last condition is mispredicted but the first wasn't? Does it just rollback all the way?
I'm talking about something like this:
if (value_in_memory == y){
// computations
if (another_val_memory == x){
//computations
}
}
Speculative execution is the regular state of execution, not a special mode that an out of order CPU enters when it sees a branch and then leaves when the branch is no longer in flight.
This is easier to see if you consider that it's not just branches that can fault, but many instructions, including those that access memory, have restrictions on their input values, etc. So any substantial out of order execution implies constant speculation, and CPUs are built around that idea.
So "nested branches" doesn't end up being special in that sense.
Now, modern CPUs have a variety of methods for quick branch misprediction recovery, faster than recovery from other types of faults1. For example they may snapshot the state of the register mapping at some branches, to allow recovery to start before the branch is at the head of the reorder buffer. Since it is not always feasible to snapshot at all branches, there might be complicated heuristics involved to decide where to take snapshots.
I mention this last part because it is one way in which nested branches might matter: when there are lots of branches in flight, you might hit some microarchitectural limits related to the tracking of these branches for recovery purposes. For more details, you can look through patents for "branch order buffer" (for Intel techniques, but there are no doubt others).
1 The basic recovery method is keep executing until the faulting instruction is the next to retire, and then throw away all younger instructions. In the context of branch mispredictions, this means you could actually suffer two or more mispredictions only the oldest of which actually takes effect: e.g., a younger branch mispredicts, and while executing up to that branch (at which point recovery can occur), another mispredict occurs, so the younger one ends up getting discarded.
(Maybe not a complete answer, but I had some of this written when #BeeOnRope posted an answer. Posting this anyway for some more links and technical details in case anyone's curious.)
Everything is always speculative until it reaches retirement and becomes non-speculative, definitely happened, part of the architectural state.
e.g. any load might fault with a bad address, any div might trap on divide by zero. See also Out-of-order execution vs. speculative execution That and What exactly happens when a skylake CPU mispredicts a branch? mention that branch mispredicts are handled specially, because they're expected to be frequent. Fast-recovery can start before a mis-predicted branch reaches retirement, unlike the behaviour for a faulting load for example. (That's part of why Meltdown is exploitable.)
So even "regular" instructions are executed speculatively before being commited, and the only distinction between them is a human-made distinction, not computer-made? I presume, then, that the CPU stores multiple, possible rollback points? For instance if I have load instructions that may lead to page faults or simply use stale values, inside a conditional branch, the CPU identifies such instructions and scenarios and saves a state for each of them? I feel like I misunderstood because this may lead to a lot of storing register states and complicated dependencies.
The retirement state is always consistent so you can always roll back to there and discard all in-flight work, e.g. if an external interrupt arrives you want to handle it without waiting for a chain of a dozen cache miss loads to all execute. When an interrupt occurs, what happens to instructions in the pipeline?
This tracking basically happens for free or is something you need to do anyway to be able to detect which instruction faulted, not just that there was a problem somewhere. (This is called "precise exceptions")
The real distinction humans can usefully make is speculation that has a real chance of being wrong during execution of non-error cases. If your code gets a bad pointer, it doesn't really matter how it performs; it's going to page-fault and that's going to be very slow compared to local OoO exec details.
You're talking about a modern out-of-order (OoO) execution (not just fetch) CPU, like modern Intel or AMD x86, high-end ARM, MIPS r10000, etc.
The front-end is in-order (with speculation down predicted paths), and so is commit (aka retirement) from the out-of-order back-end into non-speculative retirement state. (aka known-good architectural state).
The CPU uses two major structures to track instructions (or on x86, uops = parts of instructions) in the back-end. The last stage of the front-end (after fetch / decode) allocates/renames instructions and adds them into both of these structures at once.
RS = Reservation Station = scheduler: not-yet-executed instructions, waiting for an execution unit. The RS tracks dependencies and sends the oldest-ready uops to execution units that are ready.
ROB = ReOrder Buffer: not-yet-retired instructions. Instructions enter and leave in-order so it can just be a circular buffer.
Includes a flag to mark each entry as executed or not, set once the RS has sent it to an execution unit which reports success. The oldest instructions in the ROB that all have their done-executing bit set can "retire".
Also includes a flag which indicates "fault if this reaches retirement". This avoids spending time handling page faults from load instruction on the wrong path of execution (that might well have pointers into an unmapped page), for example. Either in the shadow of a branch mispredict, or just after another instruction (in program order) that should have faulted first but OoO exec got to it later.
(I'm also leaving out register-renaming onto a large physical register file.
That's the "rename" part. Allocate includes choosing which execution port an instruction will use, and reserving a load or store buffer entry for memory instructions.)
(There's also a store-buffer; stores don't write directly to L1d cache, they write to the store buffer. This makes it possible to speculatively execute stores and still roll back without them becoming visible to other cores. It also decouples cache-miss stores from execution. Once a store instruction retires, the store-buffer entry "graduates" and is eligible to commit to L1d cache, once MESI gets exclusive access to the cache line, and once memory-ordering rules are satisfied.)
Execution units detect whether an instruction should fault, or was mis-speculated and should roll back, but don't necessarily act on that until the instruction reaches retirement.
In-order retirement is the step that recovers program-order after OoO exec, including the case of exceptions of mis-speculation.
Terminology: Intel calls it "issue" when instructions are sent from the front-end into the ROB + RS. Other computer architecture people often call that "dispatch".
Sending uops from the RS to execution units is called "dispatch" by Intel, "issue" by other people.
Given the small program shown below (handcrafted to look the same from a sequential consistency / TSO perspective), and assuming it's being run by a superscalar out-of-order x86 cpu:
Load A <-- A in main memory
Load B <-- B is in L2
Store C, 123 <-- C is L1
I have a few questions:
Assuming a big enough instruction-window, will the three instructions be fetched, decoded, executed at the same time? I assume not, as that would break execution in program order.
The 2nd load is going to take longer to fetch A from memory than B. Will the later have to wait until the first is fully executed? Will the fetching of B only start after Load A is fully executed? or until when does it have to wait?
Why would the store have to wait for the loads? If yes, will the instruction just wait to be committed in the store buffer until the loads finish or after decoding it will have to sit and wait for the loads?
Thanks
Terminology: "instruction-window" normally means out-of-order execution window, over which the CPU can find ILP. i.e. ROB or RS size. See Understanding the impact of lfence on a loop with two long dependency chains, for increasing lengths
The term for how many instructions can go through the pipeline in a single cycle is pipeline width. e.g. Skylake is 4-wide superscalar out-of-order. (Parts of its pipeline, like decode, uop-cache fetch, and retirement, are wider than 4 uops, but issue/rename is the narrowest point.)
Terminology: "wait to be committed in the store buffer" store data + address gets written into the store buffer when a store executes. It commits from the store buffer to L1d at any point after retirement, when it's known to be non-speculative.
(In program order, to maintain the TSO memory model of no store reordering. A store buffer allows stores to execute inside this core out of order but still commit to L1d (and become globally visible) in-order. Executing a store = writing address + data to the store buffer.)
Can a speculatively executed CPU branch contain opcodes that access RAM?
Also what is a store buffer? and
Size of store buffers on Intel hardware? What exactly is a store buffer?
The front-end is irrelevant. 3 consecutive instructions might well be fetched in the same 16-byte fetch block, and might go through pre-decode and decode in the same cycle as a group. And (also or instead) issue into the out-of-order back-end as part of a group of 3 or 4 uops. IDK why you think any of that would cause any potential problem.
The front end (from fetch to issue/rename) processes instructions in program order. Processing simultaneously doesn't put later instructions before earlier ones, it puts them at the same time. And more importantly, it preserves the information of what program order is; that's not lost or discarded because it matters for instructions that depend on the previous one1!
There are queues between most pipeline stages, so (for example on Intel Sandybridge) instructions that pre-decode as part of a group of up-to-6 instructions might not hit the decoders as part of the same group of up-to-4 (or more with macro-fusion). See https://www.realworldtech.com/sandy-bridge/3/ for fetch, and the next page for decode. (And the uop cache.)
Executing (dispatching uops to execution ports from the out-of-order scheduler) is where ordering matters. The out-of-order scheduler has to avoid breaking single threaded code.2
Usually issue/rename is far ahead of execution, unless you're bottlenecked on the front-end. So there's normally no reason to expect that uops that issued together will execute together. (For the sake of argument, let's assume that the 2 loads you show do get dispatched for execution in the same cycle, regardless of how they got there via the front-end.)
But anyway, there's no problem here starting both loads and the store the same time. The uop scheduler doesn't know whether a load will hit or miss in L1d. It just sends 2 load uops to the load execution units in a cycle, and a store-address + store-data uop to those ports.
[load ordering]
This is the tricky part.
As I explained in an answer + comments on your last question, modern x86 CPUs will speculatively use the L2 hit result from Load B for later instructions, even though the memory model requires that this load happens after Load A.
But if no other cores write to cache line B before Load A completes, then nothing can tell the difference. The Memory-Order Buffer takes care of detecting invalidations of cache lines that were loaded from before earlier loads complete, and doing a memory-order mis-speculation pipeline flush (rollback to retirement state) in the rare case that allowing load re-ordering could change the result.
Why would the store have to wait for the loads?
It won't, unless the store-address depends on a load value. The uop scheduler will dispatch the store-address and store-data uops to execution units when their inputs are ready.
It's after the loads in program order, and the store buffer will make it even farther after the loads as far as global memory order is concerned. The store buffer won't commit the store data to L1d (making it globally visible) until after the store has retired. Since it's after the loads, they'll have also retired.
(Retirement is in-order to allow precise exceptions, and to make sure no previous instructions took an exception or were a mispredicted branch. In-order retirement allows us to say for sure that an instruction is non-speculative after it retires.)
So yes, this mechanism does ensure that the store can't commit to L1d until after both loads have taken data from memory (via L1d cache which provides a coherent view of memory to all cores). So this prevents LoadStore reordering (of earlier loads with later stores).
I'm not sure if any weakly-ordered OoO CPUs do LoadStore reordering. It is possible on in-order CPUs when a cache-miss load comes before a cache-hit store, and the CPU uses scoreboarding to avoid stalling until the load data is actually read from a register, if it still isn't ready. (LoadStore is a weird one: see also Jeff Preshing's Memory Barriers Are Like Source Control Operations). Maybe some OoO exec CPUs can also track cache-miss stores post retirement when they're known to be definitely happening, but the data just still hasn't arrived yet. x86 doesn't do this because it would violate the TSO memory model.
Footnote 1: There are some architectures (typically VLIW) where bundles of simultaneous instructions are part of the architecture in a way that's visible to software. So if software can't fill all 3 slots with instructions that can execute simultaneously, it has to fill them with NOPs. It might even be allowed to swap 2 registers with a bundle that contained mov r0, r1 and mov r1, r0, depending on whether the ISA allows instructions in the same bundle to read and write the same registers.
But x86 is not like that: superscalar out-of-order execution must always preserve the illusion of running instructions one at a time in program order. The cardinal rule of OoO exec is: don't break single-threaded code.
Anything that would violate this can only be done with checking for hazards, or speculatively with rollback upon detection of mistakes.
Footnote 2: (continued from footnote 1)
You can fetch / decode / issue two back-to-back inc eax instructions, but they can't execute in the same cycle because register renaming + the OoO scheduler has to detect that the 2nd one reads the output of the first.
The process of a Context Switch. In computing, a context switch is the
process of storing and restoring the state (more specifically, the
execution context) of a process or thread so that execution can be
resumed from the same point at a later time.
but if it is the process or thread
and resource is not shared then it must be called as Process Swap or Thread Swap
It is not swapping threads themselves or process themselves. It is swapping execution context information stored within the CPU - various CPU registers, flag bits, etc. A process contains 1+ threads, and each thread manipulates CPU state. A context switch captures the CPU state (the context) of the current running thread and pauses it, then swaps in the state of another thread so it can then resume running where it previously left off.
Keep in minds that it was called a change in process context long before there were threads. Since we are talking history, I ignore threads.
Making a process executable requires loading registers. The set of registers varies among systems but it typically includes:
General Registers
Processor Status
Use Page Table Registers
When a process ceases to be current, its register values need to be saved. When the process becomes current, its register values need to be restored.
Nearly every processor has an instruction that performs each of these tasks to allow the switch to be performed atomically.
The processor then needs to define a data structure for holding the register values. For whatever reason, this data structure was called a "Process Context Block" or PCB (some idiotic textbooks use PCB to describe something else that does not really exist).
Someone probably thought calling it a Process "Context" was a cute name. It really does not make sense in English but that is what happened.
The instructions for loading and saving then became variants of Load Process Context and Save Process Context.
but if it is the process or thread and resource is not shared then it must be called as Process Swap or Thread Swap
Swapping is an entirely different concept. So the terms "swap" and "swapping" were already taken. In the days before paging, entire processes were moved (swapped) to disk. That is why eunuchs has a "swap" partition—the name never got updated to "page partition."
In The Cache Memory Book by Jim Handy (excerpt is below), the author has the table description of MESI protocol. The table looks very unclear to me, and unfortunately the text does not help.
The first question (in green on the picture):
Is this right? -- a data block is in the cache of a CPU,
and it is in the shared state, but when the CPU reads it,
the CPU gets read miss.
How is this possible?
The second question (in purple):
Who and when create all these messages "Read miss", etc.?
(afaik, the system bus just translates messages of others)
And finally, the third question (not on the pic):
Do all these cache coherency protocols
(MSI,MESI,MOESI,Firefly,Dragon...)
maintain sequential consistency memory model?
Are there protocols that maintain other consistency models?
For the first question, while there is a data block (in shared state) in the cache, it is the wrong block (i.e., the tag does not match), so there is a cache miss. On a cache miss, the cache still needs to writeback data to memory if it is in the modified state.
For the second question, each bus is providing address and request information (read or write) to the cache; the system bus is providing this from a remote processor. The hit or miss is whether the address provided is a hit in the cache. Requests on the system bus are filtered by the remote processor's cache.
On a read by processor 1 (P1), if the P1's cache has a hit, no signal needs to be sent to the system bus. On a write by P1, if the P1's cache has a hit and the state is exclusive or modified, then no signals need to be sent to the system bus, but if state in P1's cache is shared, then the other (P0) cache must told (via the system bus) that P1 is performing a write and that any entry in P0's cache for that address must be invalidated.
(Note that "shared" state does not necessarily mean that the block of memory is present in some other cache. Most snoop-based protocols allow silent [no system bus communication] dropping of cache blocks in shared state.)
(By the way, the MESI protocol given by that book is a bit unusual. It is more common for Exclusive state to be entered by a read miss that found no other caches with the block [this allows silent update to modified on a later write] and for writes not to use write through to memory on a hit that was in shared state.)
For the third question, cache coherence protocols only address coherence not consistency. The consistency model that the hardware provides determines what kinds of activity can be buffered and completed out of order (in a way that is visible to software). E.g., it helps performance if a write miss does not force any following read hits to wait until the write notification has been seen by all remote caches, but such can violate sequential consistency.
Here is an example how sequential consistency can prevent buffering of writes:
P0 reads "B_old" at location B (cache hit), writes "A_new" to location A (a cache miss), then reads location B.
P1 reads "A_old" at location A (cache hit), writes "B_new" to location B (a cache miss), then reads location A.
If writes are buffered and reads are allowed to proceed without waiting for the preceding write to be recognized by the other cache, the P0's second read of B can read "B_old" (since it will still be a cache hit) and P1's second read of A can read "A_old" (since P0's write has not yet been seen). There is no way to construct a sequential ordering of these memory accesses, so sequential consistency would be violated.
However, if P0's second read of B waited until P0's write to A was recognized by P1 (and P1's second read of A waited until P1's write to B was recognized by P0), then a sequential ordering would be possible.
If P1 saw P0's write to A before its write to B, then sequential ordering is possible, including:
P0.read "B_old", P1.read "A_old", P0.write "A_new", P1.write "B_new", P0.read "B_new", P1.read "A_new"
P0.read "B_old", P1.read "A_old", P0.write "A_new", P0.read "B_old", P1.write "B_new", P1.read "A_new"
Note that hardware can speculate that ordering requirements are not violated and reorder the completion of memory accesses; however, it needs to be able to handle incorrect speculation (e.g., by restoration to a checkpoint). (A classic early paper on this is "Is SC + ILP = RC?" [PDF].)
There is a one-to-many mapping between cache lines and blocks of main memory. A cache line that is shared may not be the block of memory your program is looking for. What actually happened is that the block of memory your program wants, and the block of memory present in the cache, though different, have the same cache line index. Consecutive blocks of memory are mapped to consecutive cache lines, till the cache lines finish, and the subsequent blocks of memory are mapped to cache lines starting with the first cache line all over again.
I found an intuitive explanation here: https://medium.com/breaktheloop/direct-mapping-map-cache-and-main-memory-d5e4c1cbf73e