On Tue, Aug 11, 2015 at 10:50 PM, Haomai Wang <haomaiwang@xxxxxxxxx> wrote: > On Wed, Aug 12, 2015 at 6:34 AM, Yehuda Sadeh-Weinraub > <ysadehwe@xxxxxxxxxx> wrote: >> Already mentioned it on irc, adding to ceph-devel for the sake of >> completeness. I did some infrastructure work for rgw and it seems (at >> least to me) that it could at least be partially useful here. >> Basically it's an async execution framework that utilizes coroutines. >> It's comprised of aio notification manager that can also be tied into >> coroutines execution. The coroutines themselves are stackless, they >> are implemented as state machines, but using some boost trickery to >> hide the details so they can be written very similar to blocking >> methods. Coroutines can also execute other coroutines and can be >> stacked, or can generate concurrent execution. It's still somewhat in >> flux, but I think it's mostly done and already useful at this point, >> so if there's anything you could use it might be a good idea to avoid >> effort duplication. >> > > coroutines like qemu is cool. The only thing I afraid is the > complicate of debug and it's really a big task :-( > > I agree with sage that this design is really a new implementation for > objectstore so that it's harmful to existing objectstore impl. I also > suffer the pain from sync read xattr, we may add a async read > interface to solove this? > > For context switch thing, now we have at least 3 cs for one op at osd > side. messenger -> op queue -> objectstore queue. I guess op queue -> > objectstore is easier to kick off just as sam said. We can make write > journal inline with queue_transaction, so the caller could directly > handle the transaction right now. I would caution agains coroutines (fibers) esp. in a multi-threaded environment. Posix has officially obsoleted the swapcontext family of functions in 1003.1-2004 and removed it in 1003.1-2008. That's because they were notoriously non portable, and buggy. And yes you can use something like boost::context / boost::coroutine instead but they also have platform limitations. These implementations tend to abuse / turn of various platform scrutiny features (like the one for setjmp/longjmp). And on top of that many platforms don't consider alternative context so you end up with obscure bugs. I've debugged my fair share of bugs in Mordor coroutines with C++ exceptions, and errno variables (since errno is really a function on linux and it's output a pointer to threads errno is marked pure) if your coroutine migrates threads. And you need to migrate them because of blocking and uneven processor/thread distribution. None of these are obstacles that can't be solved, but added together they become a pretty long term liability. So I think long and hard about it. Qemu doesn't have some of those issues because it's uses a single thread and a much simpler C ABI that it deals with. An alternative to coroutines that goes a long way towards solving the callback spaghetti problem is futures/promises. I'm not talking of the very future model that exists in C++11 library but more along the lines that exist in other languages (like what's being done in Javascript today). There's a good implementation of it Folly (the facebook c++11 library). They have a very nice piece of documentation here to understand how they work and how they differ. That future model is very handy when dealing with the callback control flow problem. You can chain a bunch of processing steps that requires some async action, return a future and continue so on and so forth. Also, it makes handling complex error cases easy by giving you a way to skip lots of processing steps strait to onError at the end of the chain. Take a look at folly. Take a look at the expanded boost futures (they call this future continuations: http://www.boost.org/doc/libs/1_54_0/doc/html/thread/synchronization.html#thread.synchronization.futures.then ). Also, building a cut down future framework just for Ceph (or reduced set folly one) might be another option. Just an alternative. > > Anyway, I think we need to do some changes for this field. > >> Yehuda >> >> On Tue, Aug 11, 2015 at 3:19 PM, Samuel Just <sjust@xxxxxxxxxx> wrote: >>> Yeah, I'm perfectly happy to have wrappers. I'm also not at all tied >>> to the actual interface I presented so much as the notion that the >>> next thing to do is restructure the OpWQ users as async state >>> machines. >>> -Sam >>> >>> On Tue, Aug 11, 2015 at 1:05 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote: >>>> On Tue, 11 Aug 2015, Samuel Just wrote: >>>>> Currently, there are some deficiencies in how the OSD maps ops onto threads: >>>>> >>>>> 1. Reads are always syncronous limiting the queue depth seen from the device >>>>> and therefore the possible parallelism. >>>>> 2. Writes are always asyncronous forcing even very fast writes to be completed >>>>> in a seperate thread. >>>>> 3. do_op cannot surrender the thread/pg lock during an operation forcing reads >>>>> required to continue the operation to be syncronous. >>>>> >>>>> For spinning disks, this is mostly ok since they don't benefit as much from >>>>> large read queues, and writes (filestore with journal) are too slow for the >>>>> thread switches to make a big difference. For very fast flash, however, we >>>>> want the flexibility to allow the backend to perform writes syncronously or >>>>> asyncronously when it makes sense, and to maintain a larger number of >>>>> outstanding reads than we have threads. To that end, I suggest changing the >>>>> ObjectStore interface to be somewhat polling based: >>>>> >>>>> /// Create new token >>>>> void *create_operation_token() = 0; >>>>> bool is_operation_complete(void *token) = 0; >>>>> bool is_operation_committed(void *token) = 0; >>>>> bool is_operation_applied(void *token) = 0; >>>>> void wait_for_committed(void *token) = 0; >>>>> void wait_for_applied(void *token) = 0; >>>>> void wait_for_complete(void *token) = 0; >>>>> /// Get result of operation >>>>> int get_result(void *token) = 0; >>>>> /// Must only be called once is_opearation_complete(token) >>>>> void reset_operation_token(void *token) = 0; >>>>> /// Must only be called once is_opearation_complete(token) >>>>> void detroy_operation_token(void *token) = 0; >>>>> >>>>> /** >>>>> * Queue a transaction >>>>> * >>>>> * token must be either fresh or reset since the last operation. >>>>> * If the operation is completed syncronously, token can be resused >>>>> * without calling reset_operation_token. >>>>> * >>>>> * @result 0 if completed syncronously, -EAGAIN if async >>>>> */ >>>>> int queue_transaction( >>>>> Transaction *t, >>>>> OpSequencer *osr, >>>>> void *token >>>>> ) = 0; >>>>> >>>>> /** >>>>> * Queue a transaction >>>>> * >>>>> * token must be either fresh or reset since the last operation. >>>>> * If the operation is completed syncronously, token can be resused >>>>> * without calling reset_operation_token. >>>>> * >>>>> * @result -EAGAIN if async, 0 or -error otherwise. >>>>> */ >>>>> int read(..., void *token) = 0; >>>>> ... >>>>> >>>>> The "token" concept here is opaque to allow the implementation some >>>>> flexibility. Ideally, it would be nice to be able to include libaio >>>>> operation contexts directly. >>>>> >>>>> The main goal here is for the backend to have the freedom to complete >>>>> writes and reads asyncronously or syncronously as the sitation warrants. >>>>> It also leaves the interface user in control of where the operation >>>>> completion is handled. Each op thread can therefore handle its own >>>>> completions: >>>>> >>>>> struct InProgressOp { >>>>> PGRef pg; >>>>> ObjectStore::Token *token; >>>>> OpContext *ctx; >>>>> }; >>>>> vector<InProgressOp> in_progress(MAX_IN_PROGRESS); >>>> >>>> Probably a deque<> since we'll be pushign new requests and slurping off >>>> completed ones? Or, we can make token not completely opaque, so that it >>>> includes a boost::intrusive::list node and can be strung on a user-managed >>>> queue. >>>> >>>>> for (auto op : in_progress) { >>>>> op.token = objectstore->create_operation_token(); >>>>> } >>>>> >>>>> uint64_t next_to_start = 0; >>>>> uint64_t next_to_complete = 0; >>>>> >>>>> while (1) { >>>>> if (next_to_complete - next_to_start == MAX_IN_PROGRESS) { >>>>> InProgressOp &op = in_progress[next_to_complete % MAX_IN_PROGRESS]; >>>>> objectstore->wait_for_complete(op.token); >>>>> } >>>>> for (; next_to_complete < next_to_start; ++next_to_complete) { >>>>> InProgressOp &op = in_progress[next_to_complete % MAX_IN_PROGRESS]; >>>>> if (objectstore->is_operation_complete(op.token)) { >>>>> PGRef pg = op.pg; >>>>> OpContext *ctx = op.ctx; >>>>> op.pg = PGRef(); >>>>> op.ctx = nullptr; >>>>> objectstore->reset_operation_token(op.token); >>>>> if (pg->continue_op( >>>>> ctx, &in_progress_ops[next_to_start % MAX_IN_PROGRESS]) >>>>> == -EAGAIN) { >>>>> ++next_to_start; >>>>> continue; >>>>> } >>>>> } else { >>>>> break; >>>>> } >>>>> } >>>>> pair<OpRequestRef, PGRef> dq = // get new request from queue; >>>>> if (dq.second->do_op( >>>>> dq.first, &in_progress_ops[next_to_start % MAX_IN_PROGRESS]) >>>>> == -EAGAIN) { >>>>> ++next_to_start; >>>>> } >>>>> } >>>>> >>>>> A design like this would allow the op thread to move onto another task if the >>>>> objectstore implementation wants to perform an async operation. For this >>>>> to work, there is some work to be done: >>>>> >>>>> 1. All current reads in the read and write paths (probably including the attr >>>>> reads in get_object_context and friends) need to be able to handle getting >>>>> -EAGAIN from the objectstore. >>>> >>>> Can we leave the old read methods in place as blocking versions, and have >>>> them block on the token before returning? That'll make the transition >>>> less painful. >>>> >>>>> 2. Writes and reads need to be able to handle having the pg lock dropped >>>>> during the operation. This should be ok since the actual object information >>>>> is protected by the RWState locks. >>>> >>>> All of the async write pieces already handle this (recheck PG state after >>>> taking the lock). If they don't get -EAGAIN they'd just call the next >>>> stage, probably with a flag indicating that validation can be skipped >>>> (since the lock hasn't been dropped)? >>>> >>>>> 3. OpContext needs to have enough information to pick up where the operation >>>>> left off. This suggests that we should obtain all required ObjectContexts >>>>> at the beginning of the operation. Cache/Tiering complicates this. >>>> >>>> Yeah... >>>> >>>>> 4. The object class interface will need to be replaced with a new interface >>>>> based on possibly async reads. We can maintain compatibility with the >>>>> current ones by launching a new thread to handle any message which happens >>>>> to contain an old-style object class operation. >>>> >>>> Again, for now, wrappers would avoid this? >>>> >>>> s >>>>> >>>>> Most of this needs to happen to support object class operations on ec pools >>>>> anyway. >>>>> -Sam >>>>> -- >>>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >>>>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>>>> >>>>> >>> -- >>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >> -- >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > > > -- > Best Regards, > > Wheat > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Milosz Tanski CTO 16 East 34th Street, 15th floor New York, NY 10016 p: 646-253-9055 e: milosz@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
