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.
[TOC]
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 GLRenderer in display compositor in the browser/viz 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 the fence release
count. CommandBufferId is a 64-bit unsigned integer which is unique within a
CommandBufferNamespace. For example IPC command buffers are in the GPU_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. See gpu_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 is GenSyncTokenCHROMIUM which generates the
fence using CommandBufferProxyImpl::GenerateFenceSyncRelease(), and also adds
the fence to the command buffer using the internal InsertFenceSync command.
Different client processes communicate with the GPU process using channels. 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 the
GenSyncTokenUnverifiedCHROMIUM GL API serves this common case. These sync
tokens need to be verified using VerifySyncTokensCHROMIUM. Sync tokens
generated using GenSyncTokenCHROMIUM are already verified. SyncToken has a
verified_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 called sequences. 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 by Scheduler::CreateSequence which returns a
SequenceId. Tasks are posted to a sequence using Scheduler::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 through
Scheduler::ShouldYield and Scheduler::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 by SyncPointManager, and its helper
classes SyncPointOrderData and SyncPointClientState. 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 a SyncPointOrderData per sequence. Clients must create
SyncPointClientState using SyncPointManager::CreateSyncPointClientState and
identify their namespace, id, and sequence.
Waiting on a sync token is done by calling SyncPointManager::Wait() with a
sync token, order number for the wait, and a callback. The callbacks are
enqueued with the SyncPointClientState 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 the
WaitSyncTokenCHROMIUM GL command.
Sync tokens are completed when the fence is released in the GPU process by
calling SyncPointClientState::ReleaseFenceSync(). For GL command buffers, the
InsertFenceSync 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 by
SyncPointOrderData::GenerateUnprocessedOrderNumber which are stored in a queue
of unprocessed order numbers. In SyncPointManager::Wait(), the callbacks are
also enqueued with the order number of the waiting task in SyncPointOrderData
in a queue called OrderFenceQueue.
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 in
SyncPointOrderData::ValidateReleaseOrderNumber, and after completing a task in
SyncPointOrderData::FinishProcessingOrderNumber.
CHROMIUM_sync_point gpu_synchronication.md Lightweight GPU Sync Points