diff options
Diffstat (limited to 'docs/devel/multiple-iothreads.txt')
| -rw-r--r-- | docs/devel/multiple-iothreads.txt | 14 |
1 files changed, 7 insertions, 7 deletions
diff --git a/docs/devel/multiple-iothreads.txt b/docs/devel/multiple-iothreads.txt index 4865196bde..de85767b12 100644 --- a/docs/devel/multiple-iothreads.txt +++ b/docs/devel/multiple-iothreads.txt @@ -5,7 +5,7 @@ the COPYING file in the top-level directory. This document explains the IOThread feature and how to write code that runs -outside the QEMU global mutex. +outside the BQL. The main loop and IOThreads --------------------------- @@ -29,13 +29,13 @@ scalability bottleneck on hosts with many CPUs. Work can be spread across several IOThreads instead of just one main loop. When set up correctly this can improve I/O latency and reduce jitter seen by the guest. -The main loop is also deeply associated with the QEMU global mutex, which is a -scalability bottleneck in itself. vCPU threads and the main loop use the QEMU -global mutex to serialize execution of QEMU code. This mutex is necessary -because a lot of QEMU's code historically was not thread-safe. +The main loop is also deeply associated with the BQL, which is a +scalability bottleneck in itself. vCPU threads and the main loop use the BQL +to serialize execution of QEMU code. This mutex is necessary because a lot of +QEMU's code historically was not thread-safe. The fact that all I/O processing is done in a single main loop and that the -QEMU global mutex is contended by all vCPU threads and the main loop explain +BQL is contended by all vCPU threads and the main loop explain why it is desirable to place work into IOThreads. The experimental virtio-blk data-plane implementation has been benchmarked and @@ -66,7 +66,7 @@ There are several old APIs that use the main loop AioContext: Since they implicitly work on the main loop they cannot be used in code that runs in an IOThread. They might cause a crash or deadlock if called from an -IOThread since the QEMU global mutex is not held. +IOThread since the BQL is not held. Instead, use the AioContext functions directly (see include/block/aio.h): * aio_set_fd_handler() - monitor a file descriptor |