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Diffstat (limited to 'erts/emulator/internal_doc/CarrierMigration.md')
| -rw-r--r-- | erts/emulator/internal_doc/CarrierMigration.md | 134 |
1 files changed, 109 insertions, 25 deletions
diff --git a/erts/emulator/internal_doc/CarrierMigration.md b/erts/emulator/internal_doc/CarrierMigration.md index b93c11c6ec..2a9594db25 100644 --- a/erts/emulator/internal_doc/CarrierMigration.md +++ b/erts/emulator/internal_doc/CarrierMigration.md @@ -16,12 +16,12 @@ When a carrier is empty, i.e. contains only one large free block, it is deallocated. Since multiblock carriers can contain both allocated blocks and free blocks at the same time, an allocator instance might be stuck with a large amount of poorly utilized carriers if the memory -load decrease. After a peak in memory usage it is expected that not -all memory can be returned since the blocks still allocated is likely +load decreases. After a peak in memory usage it is expected that not +all memory can be returned since the blocks still allocated are likely to be dispersed over multiple carriers. Such poorly utilized carriers -can usually be reused if the memory load increase again. However, +can usually be reused if the memory load increases again. However, since each scheduler thread manages its own set of allocator -instances, and memory load is not necessarily connected to CPU load we +instances, and memory load is not necessarily correlated to CPU load, we might get into a situation where there are lots of poorly utilized multiblock carriers on some allocator instances while we need to allocate new multiblock carriers on other allocator instances. In @@ -50,13 +50,13 @@ the allocator instance manages. Free blocks in one specific carrier can be referred to from potentially every other carrier that is managed, and the amount of such references can be huge. That is, the work of removing the free blocks of such a carrier from the search -tree will be huge. One way of solving this could be to not migrate +tree will be huge. One way of solving this could be not to migrate carriers that contain lots of free blocks, but this would prevent us -from migrating carriers that potentially needs to be migrated in order +from migrating carriers that potentially need to be migrated in order to solve the problem we set out to solve. By using one data structure of free blocks in each carrier and an -allocator instance wide data structure of carriers managed by the +allocator instance-wide data structure of carriers managed by the allocator instance, the work needed in order to remove and add carriers can be kept to a minimum. When migration of carriers is enabled on a specific allocator type, we require that an allocation @@ -76,9 +76,9 @@ through a pool of carriers. In order for a carrier migration to complete, one scheduler needs to move the carrier into the pool, and another scheduler needs to take the carrier out of the pool. -The pool is implemented as a lock free, circular, double linked, +The pool is implemented as a lock-free, circular, double linked, list. The list contains a sentinel which is used as the starting point -when inserting to, or fetching from the pool. Carriers in the pool are +when inserting to, or fetching from, the pool. Carriers in the pool are elements in this list. The list can be modified by all scheduler threads @@ -108,19 +108,19 @@ all search operations need to read the content of the sentinel. If we were to modify the sentinel, the cache line containing the sentinel would unnecessarily be bounced between processors. -The `prev`, and `next` fields in the elements of the list contains the +The `prev` and `next` fields in the elements of the list contain the value of the pointer, a modification marker, and a deleted marker. Memory operations on these fields are done using atomic memory operations. When a thread has set the modification marker in a field, no-one except the thread that set the marker is allowed to modify the -field. If multiple modification markers needs to be set, we always +field. If multiple modification markers need to be set, we always begin with `next` fields followed by `prev` fields in the order following the actual pointers. This guarantees that no deadlocks will occur. When a carrier is being removed from a pool, we mark it with a thread progress value that needs to be reached before we are allowed to -modify the `next`, and `prev` fields. That is, until we reach this +modify the `next` and `prev` fields. That is, until we reach this thread progress we are not allowed to insert the carrier into the pool again, and we are not allowed to deallocate the carrier. This ensures that threads inspecting the pool always will be able to traverse the @@ -130,12 +130,12 @@ threads may have references to it via the pool. ### Migration ### -There exist one pool for each allocator type enabling migration of +There exists one pool for each allocator type enabling migration of carriers between scheduler specific allocator instances of the same allocator type. Each allocator instance keeps track of the current utilization of its -multiblock carriers. When the utilization falls below the "abandon +multiblock carriers. When the total utilization falls below the "abandon carrier utilization limit" it starts to inspect the utilization of the current carrier when deallocations are made. If also the utilization of the carrier falls below the "abandon carrier utilization limit" it @@ -146,28 +146,53 @@ Since the carrier has been unlinked from the data structure of available free blocks, no more allocations will be made in the carrier. The allocator instance putting the carrier into the pool, however, still has the responsibility of performing deallocations in -it while it remains in the pool. +it while it remains in the pool. The allocator instance with this +deallocation responsibility is here called the **employer**. -Each carrier has a flag field containing information about allocator -instance owning the carrier, a flag indicating if the carrier is in +Each carrier has a flag field containing information about the +employing allocator instance, a flag indicating if the carrier is in the pool or not, and a flag indicating if it is busy or not. When the -carrier is in the pool, the owning allocator instance needs to mark it +carrier is in the pool, the employing allocator instance needs to mark it as busy while operating on it. If another thread inspects it in order -to try to fetch it from the pool, it will abort the fetch if it is -busy. When fetching the carrier from the pool, ownership will changed -and further deallocations in the carrier will be redirected to the new -owner using the delayed dealloc functionality. +to try to fetch it from the pool, it will skip it if it is busy. When +fetching the carrier from the pool, employment will change and further +deallocations in the carrier will be redirected to the new +employer using the delayed dealloc functionality. If a carrier in the pool becomes empty, it will be withdrawn from the pool. All carriers that become empty are also always passed to its -originating allocator instance for deallocation using the delayed +**owning** allocator instance for deallocation using the delayed dealloc functionality. Since carriers this way always will be -deallocated by the allocator instance that allocated the carrier the +deallocated by the owner that allocated the carrier, the underlying functionality of allocating and deallocating carriers can remain simple and doesn't have to bother about multiple threads. In a NUMA system we will also not mix carriers originating from multiple NUMA nodes. +In short: + +* The allocator instance that created a carrier **owns** it. +* An empty carrier is always deallocated by its **owner**. +* **Ownership** never changes. +* The allocator instance that uses a carrier **employs** it. +* An **employer** can abandon a carrier into the pool. +* Pooled carriers are not allocated from. +* Deallocation in a pooled carrier is still performed by its **employer**. +* **Employment** can only change when a carrier is fetched from the pool. + +### Searching the pool ### + +To harbor real time characteristics, searching the pool is +limited. We only inspect a limited number of carriers. If none of +those carriers had a free block large enough to satisfy the allocation +request, the search will fail. A carrier in the pool can also be busy +if another thread is currently doing block deallocation work on the +carrier. A busy carrier will also be skipped by the search as it can +not satisfy the request. The pool is lock-free and we do not want to +block, waiting for the other thread to finish. + +#### Before OTP 17.4 #### + When an allocator instance needs more carrier space, it always begins by inspecting its own carriers that are waiting for thread progress before they can be deallocated. If no such carrier could be found, it @@ -176,10 +201,69 @@ it will allocate a new carrier. Regardless of where the allocator instance gets the carrier from it the just links in the carrier into its data structure of free blocks. +#### After OTP 17.4 #### + +The old search algorithm had a problem as the search always started at +the same position in the pool, the sentinel. This could lead to +contention from concurrent searching processes. But even worse, it +could lead to a "bad" state when searches fail with a high rate +leading to new carriers instead being allocated. These new carriers +may later be inserted into the pool due to bad utilization. If the +frequency of insertions into the pool is higher than successful +fetching from the pool, memory will eventually get exhausted. + +This "bad" state consists of a cluster of small and/or highly +fragmented carriers located at the sentinel in the pool. The largest free +block in such a "bad" carrier is rather small, making it unable to satisfy +most allocation requests. As the search always started at the +sentinel, any such "bad" carriers that had been left in the pool would +eventually cluster together at the sentinel. All searches first +have to skip past this cluster of "bad" carriers to reach a "good" +carrier. When the cluster gets to the same size as the search limit, +all searches will essentially fail. + +To counter the "bad cluster" problem and also ease the contention, the +search will now always start by first looking at the allocators **own** +carriers. That is, carriers that were initially created by the +allocator itself and later had been abandoned to the pool. If none of +our own abandoned carrier would do, then the search continues into the +pool, as before, to look for carriers created by other +allocators. However, if we have at least one abandoned carrier of our +own that could not satisfy the request, we can use that as entry point +into the pool. + +The result is that we prefer carriers created by the thread itself, +which is good for NUMA performance. And we get more entry points when +searching the pool, which will ease contention and clustering. + +To do the first search among own carriers, every allocator instance +has two new lists: `pooled_list` and `traitor_list`. These lists are only +accessed by the allocator itself and they only contain the allocator's +own carriers. When an owned carrier is abandoned and put in the +pool, it is also linked into `pooled_list`. When we search our +`pooled_list` and find a carrier that is no longer in the pool, we +move that carrier from `pooled_list` to `traitor_list` as it is now +employed by another allocator. If searching `pooled_list` fails, we +also do a limited search of `traitor_list`. When finding an abandoned +carrier in `traitor_list` it is either employed or moved back to +`pooled_list` if it could not satisfy the allocation request. + +When searching `pooled_list` and `traitor_list` we always start at the +point where the last search ended. This to avoid clustering +problems and increase the probability to find a "good" carrier. As +`pooled_list` and `traitor_list` are only accessed by the owning +allocator instance, they need no thread synchronization at all. + +Furthermore, the search for own carriers that are scheduled +for deallocation is now done as the last search option. The idea is +that it is better to reuse a poorly utilized carrier than to +resurrect an empty carrier that was just about to be released back to +the OS. + ### Result ### The use of this strategy of abandoning carriers with poor utilization -and reusing these in allocator instances with an increased carrier +and reusing them in allocator instances with an increased carrier demand is extremely effective and completely eliminates the problems that otherwise sometimes occurred when CPU load dropped while memory load did not. |