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
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