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Release Consistency

Release Consistency. Slides by Konstantin Shagin, 2002. The need for Relaxed Consistency Schemes. In any implementation of Sequential Consistency there should be some global control mechanism. Either of writes or reads require memory synchronization operations.

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Release Consistency

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  1. Release Consistency • Slides by Konstantin Shagin, 2002

  2. The need for Relaxed Consistency Schemes • In any implementation of Sequential Consistency there should be some global control mechanism. • Either of writes or reads require memory synchronization operations. • In most implementation writes require some kind of memory synchronization: w(x) w(y) w(x) A B

  3. barrier The Idea of Relaxed Consistency Schemes • The Relaxed Consistency Schemes are designed to allow less memory synchronization operations. • Writes can be delayed, aggregated, eliminated. • This results in less communication and therefore higher performance. w(x) w(y) w(x) A B

  4. Node n Node 1 Node 2 Mem Mem Mem network distributed shared memory Software Distributed Shared Memory page based, permissions, … single system image, shared virtual address space, …

  5. False Sharing • False sharing is a situation in which two or more processes access different variables within a page and at least one of the accesses is a write. • If only one process is allowed to write to a page at a time, false sharing leads to unnecessary communication, called the “ping-pong” effect.

  6. Understanding False Sharing x w(x) w(x) w(x) A y p p p p p p B r(y) r(y) r(y) x w(x) w(x) w(x) A y page p1 B page p2 r(y) r(y) r(y)

  7. False Sharing in Relaxed Consistency Schemes • False sharing has much smaller overhead in relaxed consistency models. • The overhead induced by false sharing can be further reduced by the the usage of multiple-writer protocols. • Multiple-writer protocols allow multiple processes to simultaneously modify their local copy of a shared page. • The modifications are merged at certain points of execution.

  8. (*) K. Gharachorloo, D. Lenoski, J. Laudon, P. Gibbons, A. Gupta, and J.L. Hennessy. Memory consistency and event ordering in scalable shared-memory multiprocessors. In Proceedings of the 17th Annual International Symposium on Computer Architecture, pages 15--26. IEEE, May 1990. Release Consistency[Gharachorloo et al. 1990, DASH]* • Introduces a special type of variables, called synchronization variablesor locks. • Locks cannot be read or written to. They can be acquired and released. For a lock L those operations are denoted by acquire(L)andrelease(L)respectively • We will say that a process that acquired a lock L but has not released it, holdsthe lock L. • No more than one process can hold a lock L. One process holds the lock while others wait.

  9. Using Release and Acquire to define execution-flow synchronization primitives • Let a set of processes release tokens by reaching the operation release in their program order. • Let another set (possibly with overlap) acquire those tokens by performing acquire operation, where acquire can proceed only when all tokens have already arrived from all releasing processes. • 2-way synchronization = lock-unlock, 1 release, 1 acquire • n-way synchronization = barrier, n releases, n acquires • PARC’s synch = k-way synchronization

  10. Model of Atomicity • A read by Pi is considered performed with respect toprocess Pk at a point in time when the issuing of a write to the same address by Pk can not affect the value returned by the read. • A write by Pi is considered performed with respect toprocess Pk at a point in time when an issued read to the same address by Pk returns the value defined by this write (or a later value). • An access is performed when it is performed with respect to all processes. • An acquire(L)by Pi is performed when Pi receives exclusive ownership of L (before any other requester). • A release(L)by Pi is performed when Pi gives away its exclusive ownership of L.

  11. Formal Definition of Release Consistency • Conditions for Release Consistency: • Before a read or write access is allowed to perform with respect to any other process, all previous acquire accesses must be performed, and • Before a release access is allowed to perform with respect to any other process, all previous read or write accesses must be performed, and • acquire and release accesses are sequentially consistent.

  12. w(x)1 r(x)0 r(x)? r(x)1 r(x)1 A rel(L1) acq(L1) B t Understanding RC From this point all processes must see the value 1 in X It is undefined what value is read here. It can be any value written by some process. Here it can be 0 or 1. 1 must be read according to rule (B), but the programmer can not be sure of it Programmer is sure that this will return 1 according to rules (C) and (A)

  13. Acquire and Release • release serves as a memory-synch operation, or a flush of the local modifications to the attention of all other processes. • According to the definition, the acquire and release operations are not only used for synchronization of execution, but also for synchronization of memory, i.e. for propagation of writes from/to other processes. • This allows to overlap the two expensive kinds of synchronization. • This turns out also simpler on the programmer from semantic point of view.

  14. Acquire and Release (cont.) • A release followed by an acquire of the same lock guarantees to the programmer that all writes previous to the release will be seen by all reads following the acquire. • The idea is to let the programmer decide which blocks of operations need be synchronized, and put them between matching pair of acquire-release operations. • In the absence of release/acquire pairs, there is no assurance that modifications will ever propagate between processes.

  15. Consistency of synchronization operations • Note the relations of the release/acquire operations to themselves also define an independent memory consistency scheme. • The rule (C) defined it to be Sequential Consistency. • There are other flavors of RC in which the consistency of synchronization operations defined to be some consistency x (e.g., Coherence). Such a memory model is denoted by RCx. • RCx is weaker than RCy if x is weaker than y. • For simplicity, we deal only with RCsc.

  16. Happened-Before relation induced by acquire/release • Redefine the happened-before relation using acquire and release instead of receive and send respectively. • We say that event e happened before event e’ (and denote it by e  e’ or e < e’) if one of the following properties holds: Processor Order: e precedes e’ in the same process Release-Acquire: e is a release and e’ is the following acquire of the same lock Transitivity: exists e’’ s.t. e < e’’ and e’’< e’

  17. w(x) r(x) w(y) w(x) r(y) r(x) w(y) r(y) acq(L2) rel(L1) rel(L2) rel(L1) t Happened-Before relation induced by acquire/release (cont.) A B acq(L1) C rel(L2) acq(L2)

  18. Competing Accesses • Two memory accesses are not synchronizedif they are independent events according to the previously defined happened-before relationship. • Two memory accesses are conflicting if they are accesses to the same memory location, and at least one of them is a write. • Conflicting accesses are said to be competing if there exists an execution in which they are not synchronized. • Competing accesses form a race condition as they may be executed concurrently.

  19. Data Races in RC • Release Consistency does not guarantee anything about propagation of updates without synchronization. Example: Initially: grades = oldDatabase; updated = false; Thread T.A. Thread Lecturer grades = newDatabase; updated = true; while (updated == false); X:=grades.gradeOf(lecturersSon); • If the modification of variable updated is passed to Lecturer, while the modification of grades is not, then Lecturer looks at the old database! • This is possible in Release Consistency, but not in Sequential Consistency.

  20. Expressiveness of Release Consistency[Gharachorloo et.al 1990] Let a properly-labeled (PL) program be such that has no competing accesses. Theorem: RCsc = SC for PL programs. Should make sure there are no data-races.

  21. w(x) w(y) w(z) rel(L) P1 x z y P2 Implementing RC • The first implementation was proposed by the inventors of RC and is called DASH. • DASH combats memory latency by pipelining writes to shared memory. • The processor is stalled only when executing a release, at which time it must wait for all its previous writes to perform.

  22. w(x) w(y) w(z) rel(L) P1 x,y,z P2 Implementing RC (cont.) • It is important to reduce the number of messages exchanges, because every message has additional fixed overhead, independent of its size. • Another implementation of RC, called Munin reduces the number of messages by buffering writes until a release.

  23. (*) John B. Carter, John K. Bennett, and Willy Zwaenepoel. Implementation and Performance of MUNIN. In Proceedings of the 13th ACM Symposium on Operating Systems Principles, pages 152--164, October 1991. Eager Release Consistency[Carter et al. 1991, Munin]* • Implementation of Release Consistency (not a new memory model). • Postpone sending modifications to the next release. • Upon a release send all accumulated modifications to all caching processes. • No memory-synchronization operations on an acquire. • Upon a miss (no local caching of the variable) get latest modification from latest modifier (need some more control to store its identity, no big deal).

  24. r(z)0 r(x)1 r(x)0 r(x)0 t Understanding ERC apply changes apply changes r(z)1 r(y)1 acq(L1) A z x,y apply changes w(x)1 w(y)1 r(z)1 B rel(L1) x,y z w(z)1 acq(L2) C rel(L2) apply changes • Release operation does not complete (is not performed) until the acknowledgements from all the processes are received.

  25. Supporting Multiple Writersin ERC • Modifications are detected by twinning. • When writing to unmodified page, its twin is created. • When releasing, the final copy of a page is compared to its twin. • The resulting difference is called a diff. • Twinning and diffing not only allow multiple writers, but also reduce communication. • Sending a diff is cheaper than sending an entire page.

  26. write P twin writable working copy release: diff  Twinning and Diffing

  27. w(x)1 w(x)1 w(y)2 w(y)2 Update-based vs. Invalidate-based • In update-based protocols the modifications are sent whereas in invalidate-based protocol only notifications of modifications are sent. Update-based Invalidate-based rel(L) rel(L) P1 P1 x:=1 “I changed x and y” y:=2 P2 P2

  28. w(x)1 w(y)2 Update-Based vs. Invalidate-Based (cont.) • Invalidations are smaller than the updates. • The bigger the coherency unit the bigger is the difference. • In invalidation-based schemes there can be significant overhead due to access misses. rel(L) P1 inv(x) x=1 y=2 get(x) get(y) inv(y) acq(L) P2 r(y) r(x)

  29. Reducing the Number of Messages • In DASH and Munin systems all processes (or all processes that cache the page) see the updates of a process. • Consider the following example of execution in Munin: w(x) rel(L) P1 w(x) acq(L) rel(L) P2 w(x) acq(L) rel(L) P3 r(x) acq(L) P4 • There are many unneeded messages. In DASH even more. • This problem exists in invalidation-based schemes as well.

  30. Reducing the Number of Messages (cont.) • Logically, however it suffices to update each processor’s copy only when it acquires L. w(x) rel(L) P1 w(x) acq(L) rel(L) P2 w(x) acq(L) rel(L) P3 r(x) acq(L) P4 • Therefore, a new algorithm, called Lazy Release Consistency (LRC) for implementing RC was proposed. • LRC is aimed at reducing both the number of messages and the amount of data exchanged.

  31. (*) P. Keleher, A. L. Cox, S. Dwarkadas, and W. Zwaenopol. Treadmarks: Distributed shared memory on standard workstations and operating systems. In Proceedings of the 1994 Winter Usenix Conference, pages 115--132, Jan. 1994. Lazy Release Consistency[Keleher et al., Treadmarks 1992]* • The idea is to postpone sending of modifications until a remote processoractually needs them. • Invalidate-based protocol • The BIG advantage: no need to get modifications that are irrelevant, because they are already masked by newer ones. • NOTE: implements a slightly more relaxed memory model than RC!

  32. Formal Definition of Lazy Release Consistency • Conditions for Lazy Release Consistency: • Before a read or write access is allowed to perform with respect to any other process, all previous acquire accesses must be performed with respect to that other process, and • Before a release access is allowed to perform with respect to any other process, all previous read or write accesses must be performed with respect to that other process, and • acquire and release accesses are sequentially consistent.

  33. r(x)? r(x)0 r(x)1 r(x)? r(x)? r(x)0 w(x)1 r(x)? r(x)? rel(L1) acq(L1) acq(L2) t Understanding the LRC Memory Model A B C • It is guaranteed that the acquirer of the same lock sees the modification that precede the release in program order.

  34. w(x)1 w(y)1 r(x)1 r(y)1 A rel(L1) acq(L1) acq(L2) rel(L2) rel(L1) acq(L2) B C t Understanding the LRC Memory Model: Transitivity • The process C sees the modification of x by A.

  35. Implementation of LRC • Satisfying the happened-before relationship between all operations is enough to satisfy LRC. • Maintenance and usage of such a detailed ordering would be expensive. • Instead, the ordering is applied to process intervals. • Intervals are segments of time in the execution of a single process. • New interval begins each time a process executes a synchronization operation.

  36. rel(L1) 1 2 3 acq(L2) acq(L1) rel(L2) acq(L3) rel(L3) rel(L1) acq(L2) 3 2 1 4 5 2 3 1 t Intervals P1 P2 P3

  37. Happened-before of Intervals • A happened before partial order is defined between intervals. • An interval i1 precedes an interval i2 according to happened-before of intervals, if all accesses in i1 precede accesses in i2 according to the happened-before of accesses.

  38. Vector Timestamps • An interval is said to be performed at a process if all interval’s accesses have been performed at that process. • Each process p has vector timestampVp that tracks which intervals have been performed at that process. • A vector timestamp consists of a set of interval indices, one per process in the system.

  39. Management of Vector Timestamps • Vector timestamps are managed like vector clocks. • send and receive events are replaced by release and acquire (of the same lock) respectively. • A lock grant message (that is sent from releaser to acquirer to give acquire the exclusive ownership) contains the current timestamp of the releaser • Just before executing a release or acquire in p: Vp[q]:= Vp[q] + 1 • A lock grant message m is time-stamped with t(m)=Vp. • Upon acquire for every q: Vp[q]:= max{ Vp[q], t(m)[q] }

  40. Vector Timestamps (cont.) • A process updates its vector timestamp at the end of an interval. Therefore during an interval the process’ timestamp does not change. • We denote the vector timestamp of process p at interval i by Vpi. • The entry for process q p is denoted by Vpi[q]. • It specifies the most recent interval of process q that has been performed at process p. • Entry Vpi[p] is always equal to i. • An interval x of process q is said to be coveredby Vpi if Vpi[q]  x

  41. Write Notices • Write noticeis an indication that a given page has been modified. • Each process keeps a table of intervals covered by it. • An entry in this table represents an interval. It contains a write notice for every page that was modified during the segment of time corresponding to the interval. • Write notices are sent in the lock grant message along with the vector timestamp of the releaser.

  42. Write Notices (cont.) • It is not necessary to send to acquirer the write notices belonging to intervals covered by its vector timestamp. • In order to let releaser know what intervals are covered by the acquirer, the acquirer sends the release its timestamp inside a lock request message. • When the releasersends a lock grant message to the acquirer, it sends only the write notices belonging to interval covered by itself, but not covered by the acquirer. • When the acquirer receives the lock grant message, it invalidates all the pages for which a write notice is included in the message.

  43. Write Notices (cont.) w(x) w(y) rel(L) acq(L) A write notices for intervals not covered by VCB write notices generate write notices request diffs diffs lock request B acq(L) r(y) x,y invalidate according to write notices

  44. Access Misses • When accessing an invalidated page, all the modifications made to it in the intervals that happened before the current interval must be obtained. • Note that this is true even if the access is a write. • A process can identify those intervals and the processes that performed the modification by the write notices it has for the page. • A write notice is saved along with the id of the process from which it was received and its vector timestamp. • How do we merge modifications performed by concurrent writers to a page?

  45. P1 P2 X Y Tracking Modifications with Multiple Writers • It is possible that several processes make modifications to different variables at the same page. • If the intervals in which the modifications are performed are independent (according to happened-before), we cannot just bring a page from one of the processes. • What should we do? Employ the twinning and diffing technique again!

  46. write P twin writable working copy release: diff  Twinning and Diffing (reminder)

  47. w(x) w(y) rel(L1) acq(L2) t Tracking Modifications with Multiple Writers (cont.) • Note that twinning and diffing not only allows multiple independent writers but also significantly reduces the amount of data sent. P1 inv(P) page P acq(L1) r(x) P2 x y inv(P) P3 rel(L2)

  48. Access Misses (cont.) • Consider the following scenario, in which P3has a miss on a page containing variables x, y and z: w(x) rel P1 inv(x) w(y) acq rel P2 inv(x,y) mod(x,y) r(z) acq P3 • When accessing z, P3sees that according to the locally stored write notices there has been two previous modifications. • They are ordered by happened before relationship therefore P3 can request both modifications from P2.

  49. Access Misses (cont.) • More generally, if processor q modified page P at its interval x, then q is guaranteed to have any diffs of P created intervals that “happened-before” the interval x. • Therefore even if diffs from multiple writers need to be retrieved, it is usually only necessary to communicate with very few processors. • How long should a process keep the diffs ? • How long should a process keep the write notices ? • Clearly, not forever! A garbage collection needs to be done…

  50. Garbage Collection • A diff needs to be retained until it is clear it will never be requested. • This happens when a diff has already been sent to every processor. • When a process sees it is running out of memory it initiates garbage collection, which is invoked at the next barrier. • Garbage collection piggybacks on the barrier to “stop the world”. Each process receives all write notices in the system and uses them to validate all of its cached pages. As a result, all write notices and diffs are discarded.

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