Changes in / [d0502a3:feacef9]


Ignore:
Location:
doc/theses/thierry_delisle_PhD/thesis
Files:
2 edited

Legend:

Unmodified
Added
Removed
  • doc/theses/thierry_delisle_PhD/thesis/fig/pivot_ring.fig

    rd0502a3 rfeacef9  
    1111        1 1 1.00 60.00 120.00
    12126 225 2475 1665 2835
     134 2 0 50 -1 0 12 0.0000 6 135 810 1665 2610 Allocated\001
    13144 2 0 50 -1 0 12 0.0000 6 165 1440 1665 2805 \\lstinline|sqe|s\001
    14 4 2 0 50 -1 0 12 0.0000 6 135 810 1665 2610 Submitted\001
    1515-6
    16166 180 3825 1620 4185
    17 4 2 0 50 -1 0 12 0.0000 6 135 810 1620 3960 Allocated\001
    18174 2 0 50 -1 0 12 0.0000 6 165 1440 1620 4155 \\lstinline|sqe|s\001
     184 2 0 50 -1 0 12 0.0000 6 135 810 1620 3960 Submitted\001
    1919-6
    20201 3 0 1 0 7 40 -1 20 0.000 1 0.0000 2475 3375 315 315 2475 3375 2790 3375
  • doc/theses/thierry_delisle_PhD/thesis/text/io.tex

    rd0502a3 rfeacef9  
    104104
    105105\subsection{Multiplexing \io: Submission}
    106 The submission side is the most complicated aspect of @io_uring@ and its design largely dictates the completion side.
    107 
    108 While it is possible to do the first steps of submission in parallel, the duration of the system call scales with number of entries submitted. The consequence is that the amount of parallelism used to prepare submissions for the next system call is limited. Beyond this limit, the length of the system call is the throughput limiting factor. I concluded from early experiments that preparing submissions seems to take about as long as the system call itself, which means that with a single @io_uring@ instance, there is no benefit in terms of \io throughput to having more than two \glspl{hthrd}. Therefore the design of the submission engine must manage multiple instances of @io_uring@ running in parallel, effectively sharding @io_uring@ instances. Similarly to scheduling, this sharding can be done privately, \ie, one instance per \glspl{proc}, or in decoupled pools, \ie, a pool of \glspl{proc} use a pool of @io_uring@ instances without one-to-one coupling between any given instance and any given \gls{proc}.
    109 
    110 \subsubsection{Pool of Instances}
    111 One approach is to have multiple shared instances. \Glspl{thrd} attempting \io operations pick one of the available instances and submits operations to that instance. Since the completion will be sent to the same instance, all instances with pending operations must be polled continuously\footnote{As will be described in Chapter~\ref{practice}, this does not translate into constant CPU usage.}. Since there is no coupling between \glspl{proc} and @io_uring@ instances in this approach, \glspl{thrd} running on more than one \gls{proc} can attempt to submit to the same instance concurrently. Since @io_uring@ effectively sets the amount of sharding needed to avoid contention on its internal locks, performance in this approach is based on two aspects: the synchronization needed to submit does not induce more contention than @io_uring@ already does and the scheme to route \io requests to specific @io_uring@ instances does not introduce contention. This second aspect has an oversized importance because it comes into play before the sharding of instances, and as such, all \glspl{hthrd} can contend on the routing algorithm.
    112 
    113 Allocation in this scheme can be handled fairly easily. Free SQEs, \ie, SQEs that aren't currently being used to represent a request, can be written to safely and have a field called @user_data@ which the kernel only reads to copy to CQEs. Allocation also requires no ordering guarantee as all free SQEs are interchangeable. This requires a simple concurrent bag. The only added complexity is that the number of SQEs is fixed, which means allocation can fail. This failure needs to be pushed up to the routing algorithm, \glspl{thrd} attempting \io operations must not be directed to @io_uring@ instances without any available SQEs. Ideally, the routing algorithm would block operations up-front if none of the instances have available SQEs.
     106The submission side is the most complicated aspect of @io_uring@ and the completion side effectively follows from the design decisions made in the submission side. While it is possible to do the first steps of submission in parallel, the duration of the system call scales with number of entries submitted. The consequence is that the amount of parallelism used to prepare submissions for the next system call is limited.
     107Beyond this limit, the length of the system call is the throughput limiting factor. I concluded from early experiments that preparing submissions seems to take about as long as the system call itself, which means that with a single @io_uring@ instance, there is no benefit in terms of \io throughput to having more than two \glspl{hthrd}. Therefore the design of the submission engine must manage multiple instances of @io_uring@ running in parallel, effectively sharding @io_uring@ instances. Similarly to scheduling, this sharding can be done privately, \ie, one instance per \glspl{proc}, in decoupled pools, \ie, a pool of \glspl{proc} use a pool of @io_uring@ instances without one-to-one coupling between any given instance and any given \gls{proc}, or some mix of the two. Since completions are sent to the instance where requests were submitted, all instances with pending operations must be polled continously\footnote{As will be described in Chapter~\ref{practice}, this does not translate into constant cpu usage.}.
     108
     109\subsubsection{Shared Instances}
     110One approach is to have multiple shared instances. \Glspl{thrd} attempting \io operations pick one of the available instances and submit operations to that instance. Since there is no coupling between \glspl{proc} and @io_uring@ instances in this approach, \glspl{thrd} running on more than one \gls{proc} can attempt to submit to the same instance concurrently. Since @io_uring@ effectively sets the amount of sharding needed to avoid contention on its internal locks, performance in this approach is based on two aspects: the synchronization needed to submit does not induce more contention than @io_uring@ already does and the scheme to route \io requests to specific @io_uring@ instances does not introduce contention. This second aspect has an oversized importance because it comes into play before the sharding of instances, and as such, all \glspl{hthrd} can contend on the routing algorithm.
     111
     112Allocation in this scheme can be handled fairly easily. Free @sqe@s, \ie, @sqe@s that aren't currently being used to represent a request, can be written to safely and have a field called @user_data@ which the kernel only reads to copy to @cqe@s. Allocation also requires no ordering guarantee as all free @sqe@s are interchangeable. This requires a simple concurrent bag. The only added complexity is that the number of @sqe@s is fixed, which means allocation can fail. This is made worst from the fact that @io_uring@ users can chain together operations, in which case all @sqe@s forming a chain must be allocated from the same instance.
     113
     114Allocation failures need to be pushed up to the routing algorithm: \glspl{thrd} attempting \io operations must not be directed to @io_uring@ instances without sufficient @sqe@s available. Furthermore, the routing algorithm should block operations up-front if none of the instances have available @sqe@s.
    114115
    115116Once an SQE is allocated, \glspl{thrd} can fill them normally, they simply need to keep track of the SQE index and which instance it belongs to.
     
    121122Finally, the completion side is much simpler since the @io_uring@ system call enforces a natural synchronization point. Polling simply needs to regularly do the system call, go through the produced CQEs and communicate the result back to the originating \glspl{thrd}. Since CQEs only own a signed 32 bit result, in addition to the copy of the @user_data@ field, all that is needed to communicate the result is a simple future~\cite{wiki:future}. If the submission side does not designate submitters, polling can also submit all SQEs as it is polling events.  A simple approach to polling is to allocate a \gls{thrd} per @io_uring@ instance and simply let the poller \glspl{thrd} poll their respective instances when scheduled. This design is especially convenient for reasons explained in Chapter~\ref{practice}.
    122123
     124<<<<<<< HEAD
     125With this pool of instances approach, the big advantage is that it is fairly flexible. It does not impose restrictions on what \glspl{thrd} submitting \io operations can and cannot do between allocations and submissions. It also can gracefully handles running out of ressources, @sqe@s or the kernel returning @EBUSY@. The down side to this is that many of the steps used for submitting need complex synchronization to work properly. The routing and allocation algorithm needs to keep track of which ring instances have available @sqe@s, block incoming requests if no instance is available, prevent barging if \glspl{thrd} are already queued up waiting for @sqe@s and handle @sqe@s being freed. The submission side needs to safely append @sqe@s to the ring buffer, make sure no @sqe@ is dropped or left pending forever, notify the allocation side when @sqe@s can be reused and handle the kernel returning @EBUSY@. All this synchronization may have a significant cost and, compare to the next approach presented, this synchronization is entirely overhead.
     126
     127\subsubsection{Private Instances}
     128Another approach is to simply create one ring instance per \gls{proc}. This alleviate the need for synchronization on the submissions, requiring only that \glspl{thrd} are not interrupted in between two submission steps. This is effectively the same requirement as using @thread_local@ variables. Since @sqe@s that are allocated must be submitted to the same ring, on the same \gls{proc}, this effectively forces the application to submit @sqe@s in allocation order\footnote{The actual requirement is that \glspl{thrd} cannot context switch between allocation and submission. This requirement means that from the subsystem's point of view, the allocation and submission are sequential. To remove this requirement, a \gls{thrd} would need the ability to ``yield to a specific \gls{proc}'', \ie, park with the promise that it will be run next on a specific \gls{proc}, the \gls{proc} attached to the correct ring.}, greatly simplifying both allocation and submission. In this design, allocation and submission form a ring partitionned ring buffer as shown in Figure~\ref{fig:pring}. Once added to the ring buffer, the attached \gls{proc} has a significant amount of flexibility with regards to when to do the system call. Possible options are: when the \gls{proc} runs out of \glspl{thrd} to run, after running a given number of threads \glspl{thrd}, etc.
     129=======
    123130With this pool of instances approach, the big advantage is that it is fairly flexible. It does not impose restrictions on what \glspl{thrd} submitting \io operations can and cannot do between allocations and submissions. It also can gracefully handle running out of resources, SQEs or the kernel returning @EBUSY@. The down side to this is that many of the steps used for submitting need complex synchronization to work properly. The routing and allocation algorithm needs to keep track of which ring instances have available SQEs, block incoming requests if no instance is available, prevent barging if \glspl{thrd} are already queued up waiting for SQEs and handle SQEs being freed. The submission side needs to safely append SQEs to the ring buffer, make sure no SQE is dropped or left pending forever, notify the allocation side when SQEs can be reused and handle the kernel returning @EBUSY@. Sharding the @io_uring@ instances should alleviate much of the contention caused by this, but all this synchronization may still have non-zero cost.
    124131
    125132\subsubsection{Private Instances}
    126133Another approach is to simply create one ring instance per \gls{proc}. This alleviate the need for synchronization on the submissions, requiring only that \glspl{thrd} are not interrupted in between two submission steps. This is effectively the same requirement as using @thread_local@ variables. Since SQEs that are allocated must be submitted to the same ring, on the same \gls{proc}, this effectively forces the application to submit SQEs in allocation order\footnote{The actual requirement is that \glspl{thrd} cannot context switch between allocation and submission. This requirement means that from the subsystem's point of view, the allocation and submission are sequential. To remove this requirement, a \gls{thrd} would need the ability to ``yield to a specific \gls{proc}'', \ie, park with the promise that it will be run next on a specific \gls{proc}, the \gls{proc} attached to the correct ring. This is not a current or planned feature of \CFA.}, greatly simplifying both allocation and submission. In this design, allocation and submission form a ring partitioned ring buffer as shown in Figure~\ref{fig:pring}. Once added to the ring buffer, the attached \gls{proc} has a significant amount of flexibility with regards to when to do the system call. Possible options are: when the \gls{proc} runs out of \glspl{thrd} to run, after running a given number of threads \glspl{thrd}, etc.
     134>>>>>>> 1830a8657cb302a89a7ca045bee06baa48b18101
    127135
    128136\begin{figure}
     
    133141\end{figure}
    134142
     143<<<<<<< HEAD
     144This approach has the advantage that it does not require much of the synchronization needed in the shared approach. This comes at the cost that \glspl{thrd} submitting \io operations have less flexibility, they cannot park or yield, and several exceptional cases are handled poorly. Instances running out of @sqe@s cannot run \glspl{thrd} wanting to do \io operations, in such a case the \gls{thrd} needs to be moved to a different \gls{proc}, the only current way of achieving this would be to @yield()@ hoping to be scheduled on a different \gls{proc}, which is not guaranteed.
     145
     146A more involved version of this approach can seem to solve most of these problems, using a pattern called \newterm{helping}. \Glspl{thrd} that which to submit \io operations but cannot do so, either because of an allocation failure or because they were migrate to a different \gls{proc} between allocation and submission, create an object representing what they wish to achieve and add it to a list somewhere. For this particular problem, one solution would be to have a list per \gls{proc} of submissions that could not be completed because the thread was moved, and another list, probably per cluster, of \glspl{thrd} that where unable to allocate enough @sqe@s. The problem with these ``solutions'' is that they are still bound by the strong coupling between \glspl{proc} and @io_uring@ instances. Imagine a simple case with two \glspl{thrd} on two \glspl{proc}, one \gls{thrd} submits an \io operation and then sets a flag, the other \gls{thrd} spins until the flag is set. If the first \gls{thrd} is preempted between allocation and submission and moves to the other \gls{proc}, the original \gls{proc} could start running the spinning \gls{thrd}. If this happens, the helping ``solution'' is for the \gls{thrd} wanting to submit is \io operation to added append an item to a list belonging to the \gls{proc} where the allocation was made. No other \gls{proc} can help the \gls{thrd} since @io_uring@ instances are strongly coupled to \glspl{proc}. However, in this case, the \gls{proc} is unable to help because it is executing the spinning \gls{thrd} mentioned when first expression this case\footnote{This particular example is completely artificial, but in the presence of many more \glspl{thrd}, it is not impossible that this problem would arise ``in the wild'' and could be difficult for users to reliably detect and avoid.}. Once in this situation, the only escape is to interrupted the execution of the \gls{thrd}, either directly or due to regular preemption, only then can the \gls{proc} take the time to handle the pending request to help. Interrupting \glspl{thrd} for this purpose is far from desireable, the cost is significant and the situation may be hard to detect. However, a more subtle reason why interrupting the \gls{thrd} is not a satisfying solution comes from the fact that the \gls{proc} is not actually using the instance it is tied to. If it were to use it, then helping could be done as part of the usage. Interrupts are needed here entirely because the \gls{proc} is tied to an instance it is not using. Therefore a more satisfying solution would be for the \gls{thrd} submitting the operation to simply notice that the instance is unused and simply go ahead and use it. This is the approach presented next.
     147
     148\subsubsection{Instance borrowing}
     149Both of the approaches presented above have higly undesirable aspects that stem from too loose coupling or too tight coupling between @io_uring@ and \glspl{proc}. In the first approach, loose coupling meant that all operations have synchronization overhead that a tighter coupling can avoid. The second approach on the other hand suffers from tight coupling causing problems when the \gls{proc} do not benefit from the coupling. While \glspl{proc} are continously issuing \io operations tight coupling is valuable since it avoids synchronization costs. However, in unlikely failure cases or when \glspl{proc} are not making use of their instance, tight coupling is no longer advantageous. A compromise between these approaches would be to allow tight coupling but have the option to revoke this coupling dynamically when failure cases arise. I call this approach ``instance borrowing''\footnote{While it looks similar to work-sharing and work-stealing, I think it is different enough from either to warrant a different verb to avoid confusion.}.
     150
     151In this approach, each cluster owns a pool of @io_uring@ instances managed by an arbiter. When a \gls{thrd} attempts to issue an \io operation, it ask for an instance from the arbiter and issues requests to that instance. However, in doing so it ties to the instance to the \gls{proc} it is currently running on. This coupling is kept until the arbiter decides to revoke it, taking back the instance and reverting the \gls{proc} to its initial state with respect to \io. This tight coupling means that synchronization can be minimal since only one \gls{proc} can use the instance at any given time, akin to the private instances approach. However, where it differs is that revocation from the arbiter means that this approach does not suffer from the potential deadlock described above.
     152
     153% Since private instances appear to work well in the easy case, an intersting option would be keep instances private while it is convenient but
     154
     155
     156% Arbitration is needed in the following cases
     157% \begin{enumerate}
     158%       \item The instance does not have sufficient @sqe@s to satisfy the request.
     159%       \item The current \gls{proc} does not currently hold an instance.
     160%       \item The current \gls{proc} has the wrong instance, this happens if the submitting \gls{thrd} context-switched between allocation and submission.
     161% \end{enumerate}
     162% In all cases
     163
     164
     165% Verbs of this design
     166
     167% Allocation: obtaining an sqe from which to fill in the io request, enforces the io instance to use since it must be the one which provided the sqe. Must interact with the arbiter if the instance does not have enough sqe for the allocation. (Typical allocation will ask for only one sqe, but chained sqe must be allocated from the same context so chains of sqe must be allocated in bulks)
     168
     169% Submition: simply adds the sqe(s) to some data structure to communicate that they are ready to go. This operation can't fail because there are as many spots in the submit buffer than there are sqes. Must interact with the arbiter only if the thread was moved between the allocation and the submission.
     170
     171% Flushing: Taking all the sqes that were submitted and making them visible to the kernel, also counting them in order to figure out what to_submit should be. Must be thread-safe with submission. Has to interact with the Arbiter if there are external submissions. Can't simply use a protected queue because adding to the array is not safe if the ring is still available for submitters. Flushing must therefore: check if there are external pending requests if so, ask the arbiter to flush otherwise use the fast flush operation.
     172
     173% Collect: Once the system call is done, it returns how many sqes were consumed by the system. These must be freed for allocation. Must interact with the arbiter to notify that things are now ready.
     174
     175% Handle: process all the produced cqe. No need to interact with any of the submission operations or the arbiter.
     176
     177
     178
     179
     180% alloc():
     181%       proc.io->in_use = true, __ATOMIC_ACQUIRE
     182%       if cltr.io.flag || !proc.io || proc.io->flag:
     183%               return alloc_slow(cltr.io, proc.io)
     184
     185%       a = alloc_fast(proc.io)
     186%       if a:
     187%               proc.io->in_use = false, __ATOMIC_RELEASE
     188%               return a
     189
     190%       return alloc_slow(cltr.io)
     191
     192% alloc_fast()
     193%       left = proc.io->submit_q.free.tail - proc.io->submit_q.free.head
     194%       if num_entries - left < want:
     195%               return None
     196
     197%       a = ready[head]
     198%       head = head + 1, __ATOMIC_RELEASE
     199
     200% alloc_slow()
     201%       cltr.io.flag = true, __ATOMIC_ACQUIRE
     202%       while(proc.io && proc.io->in_use) pause;
     203
     204
     205
     206% submit(a):
     207%       proc.io->in_use = true, __ATOMIC_ACQUIRE
     208%       if cltr.io.flag || proc.io != alloc.io || proc.io->flag:
     209%               return submit_slow(cltr.io)
     210=======
    135211This approach has the advantage that it does not require much of the synchronization needed in the shared approach. This comes at the cost that \glspl{thrd} submitting \io operations have less flexibility, they cannot park or yield, and several exceptional cases are handled poorly. Instances running out of SQEs cannot run \glspl{thrd} wanting to do \io operations, in such a case the \gls{thrd} needs to be moved to a different \gls{proc}, the only current way of achieving this would be to @yield()@ hoping to be scheduled on a different \gls{proc}, which is not guaranteed. Another problematic case is that \glspl{thrd} that do not park for long periods of time will delay the submission of any SQE not already submitted. This issue is similar to fairness issues which schedulers that use work-stealing mentioned in the previous chapter.
    136 
    137 
     212>>>>>>> 1830a8657cb302a89a7ca045bee06baa48b18101
     213
     214%       submit_fast(proc.io, a)
     215%       proc.io->in_use = false, __ATOMIC_RELEASE
     216
     217% polling()
     218%       loop:
     219%               yield
     220%               flush()
     221%               io_uring_enter
     222%               collect
     223%               handle()
    138224
    139225\section{Interface}
Note: See TracChangeset for help on using the changeset viewer.