Sync Objects and Fences - Praise for OpenGL ES 30 Programming Guide - page 358

the server has completed execution of previously submitted commands.
Consider another example where multiple OpenGL ES contexts (each current
to a different thread) are sharing objects. To synchronize correctly between
these contexts, it is important that commands from context A be issued to
the server before commands from context B, which depends on OpenGL
ES state modified by context A. The glFlush command is used to flush any
pending commands in the current OpenGL ES context and issue them to the
server. Note that glFlush only issues the commands to the server; it does
not wait for them to complete. If the client requires that the commands be
completed, the glFinish command should be used. We do not recommend
using glFinish unless absolutely necessary. Because glFinish does not
return until all queued commands in the context have been completely
processed by the server, calling glFinish can adversely impact performance
by forcing the client and the server to synchronize their operations.

Why Use a Sync Object?

OpenGL ES 3.0 introduces a new feature, called a fence, that provides a
way for the application to inform the GPU to wait until a set of OpenGL
ES operations have finished executing before queuing up more for
execution. You can insert a fence command into the GL command stream
and associate it with a sync object to be waited on.

If we compare using sync objects to the glFinish command, sync objects
are more efficient, as you can wait on partial completions of the GL
command stream. By comparison, calling the glFinish command may
reduce the performance of your applications, as this command will empty
the graphics pipeline.

Creating and Deleting a Sync Object

To insert a fence command to the GL command stream and create a sync
object, you can call the following function:

GLsync glFenceSync (GLenum condition , GLbitfield flags )

condition specifies the condition that must be met to signal the sync
object; must be GL_SYNC_GPU_COMMANDS_COMPLETE

specifies a bit-wise combination of flags to control the
behavior of the sync object; must be zero presently

flags