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Previously, Xlib/XCB used XCB's Xlib lock to prevent XCB from sending requests between calls to Xlib's LockDisplay and UnlockDisplay macros. Xlib/XCB then sent all of its requests using XCB's xcb_send_request, and had to flush its requests when unlocking the display. XCB 1.2 adds a new socket handoff mechanism, xcb_take_socket. Replace much of the existing Xlib/XCB implementation with the use of xcb_take_socket to take ownership of the write side of the X connection socket, and a return_socket callback which writes any outstanding requests with xcb_writev. This approach allows Xlib/XCB to use the same buffering as traditional Xlib did. In particular, programs which use Xlib/XCB and never make XCB calls will never need to hand the socket back to XCB, and vice versa. This allows us to discard large quantities of synchronization code from Xlib/XCB, together with the synchronization bugs present in that code. Several test cases which previously failed now work perfectly, including multi-threaded ico. In addition, the infamous locking correctness assertions, triggered when double-locking or when unlocking without a previous lock, no longer exist, because Xlib/XCB no longer has any reason to care more about application locking than traditional Xlib does. Furthermore, the handoff approach provides great improvements to performance. Results from x11perf's XNoOp test, which represented the worst case for the lock-based Xlib/XCB: Traditional Xlib: average 19100000/sec Lock-based Xlib/XCB: average 3350000/sec Handoff-based Xlib/XCB: average 17400000/sec Thus, for no-ops, the handoff mechanism provides more than a 4x speedup to Xlib/XCB, bringing Xlib/XCB within 9% of traditional Xlib no-op performance. Of course, real-world workloads do not use no-op, so your mileage may vary. In particular, since no-ops represent the worst case, we expect real workloads to more closely match the performance of traditional Xlib. While removing synchronization code, we changed _XReply to not drop any locks when calling xcb_wait_for_reply; previously, we had to carefully avoid a deadlock between the Display lock and the XCB Xlib lock. Holding the locks reduces implementation complexity and should not impact applications. Commit by Jamey Sharp and Josh Triplett. XCB's handoff mechanism inspired by Keith Packard.
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