Linked by Hadrien Grasland on Thu 19th May 2011 21:31 UTC
Hardware, Embedded Systems Having read the feedback resulting from my previous post on interrupts (itself resulting from an earlier OSnews Asks item on the subject), I've had a look at the way interrupts work on PowerPC v2.02, SPARC v9, Alpha and IA-64 (Itanium), and contribute this back to anyone who's interested (or willing to report any blatant flaw found in my posts). I've also tried to rework a bit my interrupt handling model to make it significantly clearer and have it look more like a design doc and less like a code draft.
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RE[2]: Pop-up threads
by Alfman on Sat 21st May 2011 00:28 UTC in reply to "RE: Pop-up threads"
Alfman
Member since:
2011-01-28

"If things which can run in parallel run in parallel, you get better scalability than with an all-sequential model like async."


If those are your assumptions, then I can understand your conclusions, but you're assumptions are pretty weak. Hypothetically I could perform nearly any computation in parallel by dividing it into threads. But doing so does not imply better performance. That depends on the ratio of cpu work versus synchronization overhead.

If I create a MT ray tracer on an 8 core processor, which will perform the best?
A) a new thread for each pixel
B) a new thread for each line
C) 8 threads, processing individual lines from a dispatch queue.
D) 8 threads, processing 1/8th of the image at a time

The answer here is obviously not A or B. For performance it never makes any sense to have more threads than cores.

C and D are probably close, but C could win since some cores might finish their work before others, leaving them idle.

"They are cheap (or more exactly can be made so). Why not use them ?"

The point is that it doesn't make sense to use them just because you can, it makes sense to use them when the tradeoffs point in that direction.

"If the task is not CPU-bound, and we consider IO waiting times that are typically orders of magnitude larger than instruction execution times..."

Fine, then your report should say that: the overhead of threads is outweighed by I/O bound factors.

"If the task is CPU-bound, threads offer significantly better performance."

This isn't automatically true. It depends on the work/overhead ratio, especially small workloads will suffer greatly when multithreaded compared to async with no overhead.

It's additionally possible to run a separate async handler on each core using cpu affinity such that multiple async requests can run in parallel with no synchronization overhead at all.


"If so much synchronization is needed that it has a significant impact on interrupt processing speed, it is indeed better to use async."

"Significant" is a judgment call I leave to you. I just object to your claim that threaded is more scalable.


"Which implies going back to the kernel, creating a thread, and waiting until the scheduler dares to run it. Why not have the kernel just do it right away?"

Firstly, most I/O operations will not need threads in the first place, async is already sufficient, why go through the overhead when it's not needed. Secondly if you do have a CPU bound operation, then the cost of a syscall should be fairly negligible. Thirdly, the cost of starting a microthread should be no worse in this scenario than when you start it by default (although I realize this is not strictly true for you since you're using a microkernel which is subject to additional IPC overhead).


"Again, how can processing events sequentially be any more scalable than processing them in parallel?"

You can't just put it in a thread and expect it to run faster.

The overhead for many small threads adds up compared to bigger threads which do more work.
If it costs 3000 cycles to send a thread to another CPU and another 3000 cycles to retrieve the results (thread creation+data transfer), then computations under 12000 cycles are probably better off being run serially on one cpu. Not only is it faster in real time, but it frees the bus for useful work on the other cpu.

You may shave off cycles here and there, but have you ever asked the question "do my tasks actually need threads?"


"Drivers have to explicitely make a choice between using a threaded or an async model."

Ok, but will you get the full async performance benefits if your trying to emulate it inside a threaded model? And it still bothers me that you characterized the async model as worse for performance.

"Yup, that's the whole point of using a threaded model at all."

Forgive me if I'm pointing out something you already considered, but my point was the fact that you are pre-emptively acknowledging the interrupt means that the drivers have to protect themselves from re-entrant (or double threaded in your model) IRQ behavior which would not normally be possible otherwise.

In other words, the implicit ack requires drivers to have more synchronization than would otherwise be necessary if they explicitly re-enabled their interrupt when ready. It may not be a big deal, but it's still several hundred/thousand cpu cycles wasted every IRQ.

The thing that baffles me about your model for drivers, is that it emphasizes threads which will rarely, if ever, be used without a mutex to serialize them again. If one driver forgets a mutex and accesses a device simultaneously from multiple CPUs, it is extremely likely to be a bug rather than a deliberate action.

Edited 2011-05-21 00:29 UTC

Reply Parent Score: 1

RE[3]: Pop-up threads
by Neolander on Sat 21st May 2011 09:24 in reply to "RE[2]: Pop-up threads"
Neolander Member since:
2010-03-08

If those are your assumptions, then I can understand your conclusions, but you're assumptions are pretty weak. Hypothetically I could perform nearly any computation in parallel by dividing it into threads. But doing so does not imply better performance. That depends on the ratio of cpu work versus synchronization overhead.

If I create a MT ray tracer on an 8 core processor, which will perform the best?
A) a new thread for each pixel
B) a new thread for each line
C) 8 threads, processing individual lines from a dispatch queue.
D) 8 threads, processing 1/8th of the image at a time

The answer here is obviously not A or B. For performance it never makes any sense to have more threads than cores.

C and D are probably close, but C could win since some cores might finish their work before others, leaving them idle.

Good point, I should try to avoid having more running threads than there are available CPUs. I like this raytracer example a lot, except that I'd argue that in this case the synchronization overhead is close to zero since IIRC raytracers render individual pixels independently. Threading overhead here will be mostly caused by the cost of context switches and thread creation/anihilation. But that's totally nitpicking.

No, what leaves me wondering here is, how I/O calls can be managed in this model.

That's because you see, if you have 16 running threads, a 8-core processor, and suddenly 4 threads have to do I/O, then they can simply use blocking calls without worrying because 4 threads from the 8 ones that weren't running will take their place.

Now, if you have 8 running threads and 8 tasks waiting in the dispatch queue, and suddenly 4 threads have to do I/O, then for optimal performance you need to replace these 4 threads with 4 new number-crunching threads from the dispatch queues' task.

In this case, we have some overhead :
1/Threads must synchronize with each other so that they don't start 4 new threads with the same pending task in a textbook example of a race
2/4 new threads must be created, which implies some context switching overhead that could have been avoided if all threads were created during a time where the kernel runs.
3/After a while, the I/O task is completed, and our 4 old threads are ready to run again. We are now dealing with 12 threads, so at this point, 4 threads must be silenced for optimal performance. How do we choose them ?

To solve problems 1/ and 2/, having lots of threads created from the start sounds like the most sensible option, if threads are sufficiently cheap. Well, I have to define "cheap", isn't it ? Let's do it : threads that are not running cost CPU time when they are created and take some room in RAM because of OS management structures (a few bytes, negligible on modern computers) but also because of their stack. The CPU time cost can't be avoided, so creating lots of threads is only interesting for tasks which run for a sufficiently long time, as you point out. But the RAM cost can totally be avoided for threads that have never run, by having the stack allocated at run time in a COW-like mechanism.

An interesting experiment to do would be a mix of my two pop-up threads models where among the threads created, only <insert number of cores here> are allowed to run from the start, and when a thread blocks the scheduler automatically picks another one from the queue.

3/ is more interesting. I think the older thread should run first, because otherwise the number of threads which have run (and, as such, have a stack) is going to grow, in a waste of RAM.

Fine, then your report should say that: the overhead of threads is outweighed by I/O bound factors.

The thing is, not all interrupts are equal from that point of view. As an example, processing keyboard or mouse interrupts is nearly pure I/O. But processing a stream of multimedia data can require significantly more number crunching, to the point where the amount of I/O is negligible, especially if DMA is being used.

It's additionally possible to run a separate async handler on each core using cpu affinity such that multiple async requests can run in parallel with no synchronization overhead at all.

Sorry, did not understand this one. Can you please elaborate ?

Firstly, most I/O operations will not need threads in the first place, async is already sufficient, why go through the overhead when it's not needed.

Indeed, for pure I/O operations, it makes more sense to use pure async. As an example, an HDD driver is totally something which should be written in an async fashion, with a message queue.

I have to do more design work on this side.

Secondly if you do have a CPU bound operation, then the cost of a syscall should be fairly negligible.

Very fair point.

Thirdly, the cost of starting a microthread should be no worse in this scenario than when you start it by default (although I realize this is not strictly true for you since you're using a microkernel which is subject to additional IPC overhead).

There is a difference in design philosophy, though : who carries the burden of creating and dispatching threads ? In one case, it's a third-party developer, in the other case it's the operating system. I'd spontaneously believe that the second solution would result in more people considering the threaded option, since it costs them less.

You may shave off cycles here and there, but have you ever asked the question "do my tasks actually need threads?"

I've a bit went the other way around : threads sounded like the cleanest multicore-friendly way to implement the model of processes providing services to each other which I aim at, and then I've tried to apply this model to interrupt processing and drivers.

Ok, but will you get the full async performance benefits if your trying to emulate it inside a threaded model?

If by performance benefits you mention the fact that async has only one task running at the time and as such doesn't have to care about synchronization and that pending tasks cost is kept minimal, then yes this model may provide that. If you wonder if the total cost of starting an interrupt processing job is better or worse than in a pure async model where you just send a message to the dispatcher's entry pipe, then I can't answer for sure, but I think that both models have advantages and drawbacks.

Having an external dispatcher in the driver process implies a slight kernel-driver communication protocol overhead which should make emulated async more powerful when the driver is initially idle. On the other hand, when the driver is initially busy, sending a message in a pipe is faster than creating an inactive pop-up thread.

And it still bothers me that you characterized the async model as worse for performance.

Indeed, that part was bad. I wrote it too late in the evening, and didn't take the time to write an in-depth comparison of both. I didn't talk about performance, but scalability though.

The thing that baffles me about your model for drivers, is that it emphasizes threads which will rarely, if ever, be used without a mutex to serialize them again. If one driver forgets a mutex and accesses a device simultaneously from multiple CPUs, it is extremely likely to be a bug rather than a deliberate action.

The assumption behind this is that in many situations, the order in which things are processed only matters in the end, when the processing results are sent to higher-level layers. Like rendering a GUI : you can render individual controls in parallel, blit them in parallel as long as they don't overlap, it's only at the time of the final copy to the framebuffer that synchronization is needed.

I don't know how much it holds true for interrupt processing, though.

Reply Parent Score: 1

RE[4]: Pop-up threads
by Alfman on Sat 21st May 2011 11:14 in reply to "RE[3]: Pop-up threads"
Alfman Member since:
2011-01-28

I'm suffering from exhaustion at this point, but I'll try to respond quickly:


As to the number of threads versus cores, you could distinguish between "scheduled threads" and "active threads". To minimize thrashing, a scheduled thread should not start until there is a free core. Unless of course there is a deadline/priority scheduler with a pending deadline on a higher priority thread.

Hmm, I think you've got it figured out already.


"The thing is, not all interrupts are equal from that point of view. As an example, processing keyboard or mouse interrupts is nearly pure I/O."

I don't think the keyboard will be a problem.

"But processing a stream of multimedia data can require significantly more number crunching, to the point where the amount of I/O is negligible, especially if DMA is being used."

A surprising amount of IO can be done without much CPU effort, copying files between disks, sending files over network (with adapters supporting multi-address DMA and checksum). The CPU can get away with communicating memory addresses instead of actual bytes.

However, in cases where the driver has to do more with the data (1000s of cycles), then there is no doubt a thread is necessary to prevent latency of other pending requests.

Interestingly the x86 interrupt hardware already has a form of prioritization without threads. Low IRQs can interrupt even when higher IRQs haven't been acked yet, the inverse is not true. So even long running IRQs at low priority are theoretically ok since they'll never interfere with high priority IRQs.

...but the problem is that the driver code is no longer atomic, and therefor must be re-entrant. It becomes impossible to use a shared critical resource from two handlers. The second interrupt obviously cannot block, since doing so would block the first interrupt holding the critical resource as well. So, while this has it's place in extremely low latency applications, I doubt it's helpful for you.


"Sorry, did not understand this one. Can you please elaborate ?"


Consider an async model on a single processor. Now multiply that by X processors running X async handlers.
Now, you can map each device to raise interrupts on a specific CPU, also give the driver affinity to that CPU. Now when an interrupt for the device occurs, the driver can safely assume an async model for all internal resources without any synchronization.
Assuming the devices are well distributed (statically) between CPUs, then they'll be serviced in parallel.

The requirement to route the IRQs to a specific CPU may not be strictly necessary, but it simplifies the example and reduces latency caused by transferring the request. This could even be true for a threaded model?



"who carries the burden of creating and dispatching threads ? In one case, it's a third-party developer, in the other case it's the operating system. I'd spontaneously believe that the second solution would result in more people considering the threaded option, since it costs them less."

Well, there's no denying that last point. Unless you allow userspace threads which don't require a syscall?


"I've a bit went the other way around : threads sounded like the cleanest multicore-friendly way to implement the model of processes providing services to each other which I aim at, and then I've tried to apply this model to interrupt processing and drivers."

I think a clean & consistent design is a good reason to use light threads. However, part of the difficulty in providing an async userspace interface under linux (which is still a mess) was the fact that the kernel needed threads internally to handle blocking calls, even though there was no associated userspace threads to block. It's doable but not clean.


You hit some good points further down, but I'm so tired...I'd better not tackle them today.

Edited 2011-05-21 11:20 UTC

Reply Parent Score: 1

RE[4]: Pop-up threads
by Alfman on Sun 22nd May 2011 19:18 in reply to "RE[3]: Pop-up threads"
Alfman Member since:
2011-01-28

Neolander,

"If by performance benefits you mention the fact that async has only one task running at the time and as such doesn't have to care about synchronization and that pending tasks cost is kept minimal, then yes this model may provide that."

The thing is, you are going to have async and threaded drivers running side by side. This implies that even the async drivers will need to use synchronization when accessing shared resources.

I throw out this as an example:

Two userpace apps read from two different files. In the threaded model, this results in two user+kernel threads blocking in the file system driver, which itself has blocked in the block device driver.

Ignoring all the synchronization needed in the FS driver (and writes), the block driver (or the cache handler) must/should create a mutex around the blocks being read so that other threads requesting the same blocks are blocked until it is read. I think such a structure would require at least two mutexes, one for the specific blocks being read, and another for the structure itself.

Therefor a thread reading a cached block would only need to synchronize against one mutex, find it's data, and return immediately.

A thread reading an uncached block would synchronize against the structure mutex, latch onto a new or existing block read mutex, queue the disk request (if new) and release the structure mutex.

This way the structure mutex is only ever held momentarily, and the read mutex is held until the disk reads a block. After which all blocked read threads can resume.

I expect that this is more or less what you'll be writing?


Now, my point about async drivers is that zero synchronization is needed. It's true, that requests to this driver will be forced to be serialized.

However:
1) Many drivers, like the one described above, need a mutex to serialize requests anyways. (This could be mitigated by dividing the disk/structure into 4 regions so that concurrent threads are less likely to bump into each other, however then you need another mutex to serialize the requests to disk since IO to one device cannot be executed from two CPUs simultaneously).

2) If the driver's primary role is scheduling DMA IO and twiddling memory structures with very little computation, then these tasks are not really suitable for parallelization since the overhead of synchronization exceeds the cost of just doing the work serially.


"The assumption behind this is that in many situations, the order in which things are processed only matters in the end, when the processing results are sent to higher-level layers. Like rendering a GUI..."


Yes, I won't deny that some layers will benefit from parallelism. However, propagating that parallelism into drivers which are fundamentally serial in nature will make those drivers more complex and could even slow them down. These drivers will require thread synchronizations when an async model could handle it's state without synchronizations (more on this later for the other poster).

I'd like to emphasize that I'm not arguing against the threaded model, particularly when they make or break the design of the paradigm. I'm just trying to suggest that sometimes there are cases where the best MT implementation performs worse than the best ST implementation, and device drivers may be one of those cases.

Reply Parent Score: 1