Chrome uses a mechanism known as “sync tokens” to synchronize different command buffers in the GPU process. This document discusses the internals of the sync token system.
In Chrome, multiple processes, for example browser and renderer, submit work to the GPU process asynchronously in command buffer. However, there are dependencies between the work submitted by different processes, such as SkiaRenderer in the display compositor in the GPU process rendering a tile produced by the raster worker in the renderer process.
Sync tokens are used to synchronize the work contained in command buffers without waiting for the work to complete. This improves pipelining, and with the introduction of GPU scheduling, allows prioritization of work. Although originally built for synchronizing command buffers, they can be used for other work in the GPU process.
Sync tokens are represented by a namespace, identifier, and thefence release count.CommandBufferId is a 64-bit unsigned integer which is unique within aCommandBufferNamespace. For example IPC command buffers are in theGPU_IO CommandBufferNamespace, and are identified by CommandBufferId with process id as the MSB and IPC route id as the LSB.
The fence release count marks completion of some work in a command buffer. Note: this is CPU side work done that includes command decoding, validation, issuing GL calls to the driver, etc. and not GPU side work. Seegpu_synchronication.md for more information about synchronizing GPU work.
Fences are typically generated or inserted on the client using a sequential counter. The corresponding GL API isGenSyncTokenCHROMIUM which generates the fence usingCommandBufferProxyImpl::GenerateFenceSyncRelease(), and also adds the fence to the command buffer using the internalInsertFenceSync command.
Different client processes communicate with the GPU process usingchannels. A channel wraps around a message pipe which doesn't provide ordering guarantees with respect to other pipes. For example, a message from the browser process containing a sync token wait can arrive before the message from the renderer process that releases or fulfills the sync token promise.
To prevent the above problem, client processes must verify sync tokens before sending to another process. Verification involves a synchronous nop IPC message,GpuChannelMsg_Nop, to the GPU process which ensures that the GPU process has read previous messages from the pipe.
Sync tokens used within a process do not need to be verified, and theGenSyncTokenUnverifiedCHROMIUM GL API serves this common case. These sync tokens need to be verified usingVerifySyncTokensCHROMIUM. Sync tokens generated usingGenSyncTokenCHROMIUM are already verified.SyncToken has averified_flush bit that guards against accidentally sending unverified sync tokens over IPC.
In the GPU process, command buffers are organized into logical streams of execution that are calledsequences. Within a sequence tasks are ordered, but are asynchronous with respect to tasks in other sequences. Dependencies between tasks are specified as sync tokens. For IPC command buffers, this implies flush ordering within a sequence.
A sequence can be created byScheduler::CreateSequence which returns aSequenceId. Tasks are posted to a sequence usingScheduler::ScheduleTask. Typically there is one sequence per channel, but sometimes there are more like raster, compositor, and media streams in renderer's channel.
The scheduler also provides a means for co-operative scheduling throughScheduler::ShouldYield andScheduler::ContinueTask. These allow a task to yield and continue once higher priority work is complete. Together with the GPU scheduler, multiple sequences provide the means for prioritization of UI work over raster prepaint work.
Sync tokens are managed in the GPU process bySyncPointManager, and its helper classesSyncPointOrderData andSyncPointClientState.SyncPointOrderData holds state for a logical stream of execution, typically containing work of multiple command buffers from one process.SyncPointClientState holds sync token state for a client which generated sync tokens, typically an IPC command buffer.
GPU scheduler maintains aSyncPointOrderData per sequence. Clients must create SyncPointClientState usingSyncPointManager::CreateSyncPointClientState and identify their namespace, id, and sequence.
Waiting on a sync token is done by callingSyncPointManager::Wait() with a sync token, order number for the wait, and a callback. The callbacks are enqueued with theSyncPointClientState of the target with the release count of the sync token. The scheduler does this internally for sync token dependencies for scheduled tasks, but the wait can also be performed when running theWaitSyncTokenCHROMIUM GL command.
Sync tokens are completed when the fence is released in the GPU process by callingSyncPointClientState::ReleaseFenceSync(). For GL command buffers, theInsertFenceSync command, which contains the release count generated in the client, calls this when executed in the service. This issues callbacks and allows waiting command buffers to resume their work.
Correctness of waits and releases basically amounts to checking that there are no indefinite waits because of broken promises or circular wait chains. This is ensured by associating an order number with each wait and release and maintaining the invariant that the order number of release is less than or equal to the order number of wait.
Each task is assigned a global sequential order number generated bySyncPointOrderData::GenerateUnprocessedOrderNumber which are stored in a queue of unprocessed order numbers. InSyncPointManager::Wait(), the callbacks are also enqueued with the order number of the waiting task inSyncPointOrderData in a queue calledOrderFenceQueue.
SyncPointOrderData maintains the invariant that all waiting callbacks must have an order number greater than the sequence's next unprocessed order number. This invariant is checked when enqueuing a new callback inSyncPointOrderData::ValidateReleaseOrderNumber, and after completing a task inSyncPointOrderData::FinishProcessingOrderNumber.
CHROMIUM_sync_pointgpu_synchronication.mdLightweight GPU Sync Points