Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

[Suparna Bhattacharya]

On Wed, Feb 21, 2007 at 10:13:55PM +0100, Ingo Molnar wrote:
> this is the v3 release of the syslet/threadlet subsystem:
> 
>    http://redhat.com/~mingo/syslet-patches/
> 
> This release came a few days later than i originally wanted, because 
> i've implemented many fundamental changes to the code. The biggest 
> highlights of v3 are:
> 
>  - "Threadlets": the introduction of the 'threadlet' execution concept.
> 
>  - syslets: multiple rings support with no kernel-side footprint, the 
>    elimination of mlock() pinning, no async_register/unregister() calls 
>    needed anymore and more.
> 
> "Threadlets" are basically the user-space equivalent of syslets: small 
> functions of execution that the kernel attempts to execute without 
> scheduling. If the threadlet blocks, the kernel creates a real thread 
> from it, and execution continues in that thread. The 'head' context (the 
> context that never blocks) returns to the original function that called 
> the threadlet. Threadlets are very easy to use:
> 
> long my_threadlet_fn(void *data)
> {
> 	char *name = data;
> 	int fd;
> 
> 	fd = open(name, O_RDONLY);
> 	if (fd < 0)
> 		goto out;
> 
> 	fstat(fd, &stat);
> 	read(fd, buf, count)
> 	...
> 
> out:
> 	return threadlet_complete();
> }
> 
> 
> main()
> {
> 	done = threadlet_exec(threadlet_fn, new_stack, &user_head);
> 	if (!done)
> 		reqs_queued++;
> }
> 
> There is no limitation whatsoever about how a threadlet function can 
> look like: it can use arbitrary system-calls and all execution will be 
> procedural. There is no 'registration' needed when running threadlets 
> either: the kernel will take care of all the details, user-space just 
> runs a threadlet without any preparation and that's it.
> 
> Completion of async threadlets can be done from user-space via any of 
> the existing APIs: in threadlet-test.c (see the async-test-v3.tar.gz 
> user-space examples at the URL above) i've for example used a futex 
> between the head and the async threads to do threadlet notification. But 
> select(), poll() or signals can be used too - whichever is most 
> convenient to the application writer.
> 
> Threadlets can also be thought of as 'optional threads': they execute in 
> the original context as long as they do not block, but once they block, 
> they are moved off into their separate thread context - and the original 
> context can continue execution.
> 
> Threadlets can also be thought of as 'on-demand parallelism': user-space 
> does not have to worry about setting up, sizing and feeding a thread 
> pool - the kernel will execute the workload in a single-threaded manner 
> as long as it makes sense, but once the context blocks, a parallel 
> context is created. So parallelism inside applications is utilized in a 
> natural way. (The best place to do this is in the kernel - user-space 
> has no idea about what level of parallelism is best for any given 
> moment.)
> 
> I believe this threadlet concept is what user-space will want to use for 
> programmable parallelism.
> 
> [ Note that right now there's a pair of system-calls: sys_threadlet_on() 
>   and sys_threadlet_off() that demarks the beginning and the end of a 
>   syslet function, which enter the kernel even in the 'cached' case - 
>   but my plan is to do these two system calls via a vsyscall, without 
>   having to enter the kernel at all. That will reduce cached threadlet 
>   execution NULL-overhead to around 10 nsecs - making it essentially 
>   zero. ]
> 
> Threadlets share much of the scheduling infrastructure with syslets.
> 
> Syslets (small, kernel-side, scripted "syscall plugins") are still 
> supported - they are (much...) harder to program than threadlets but 
> they allow the highest performance. Core infrastructure libraries like 
> glibc/libaio are expected to use syslets. Jens Axboe's FIO tool already 
> includes support for v2 syslets, and the following patch updates FIO to 

Ah, glad to see that - I was wondering if it was worthwhile to try adding
syslet support to aio-stress to be able to perform some comparisons.
Hopefully FIO should be able to generate a similar workload, but I haven't
tried it yet so am not sure. Are you planning to upload some results
(so I can compare it with patterns I am familiar with) ?

Regards
Suparna

> the v3 API:
> 
>    http://redhat.com/~mingo/syslet-patches/fio-syslet-v3.patch
> 
> Furthermore, the syslet code and API has been significantly enhanced as 
> well:
> 
>  - support for multiple completion rings has been added
> 
>  - there is no more mlock()ing of the completion ring(s)
> 
>  - sys_async_register()/unregister() has been removed as it is not 
>    needed anymore. sys_async_exec() can be called straight away.
> 
>  - there is no kernel-side resource used up by async completion rings at 
>    all (all the state is in user-space), so an arbitrary number of
>    completion rings are supported.
> 
> plus lots of bugs were fixed and a good number of cleanups were done as 
> well. The v3 code is ABI-incompatible with v2, due to these fundamental 
> changes.
> 
> As always, comments, suggestions, reports are welcome.
> 
> 	Ingo

-- 
Suparna Bhattacharya (suparna@xxxxxxxxxx)
Linux Technology Center
IBM Software Lab, India

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