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X-Received-From: 162.221.156.55 Subject: [Qemu-devel] [PATCH v3 1/3] xen-block: improve batching behaviour X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Stefano Stabellini , Tim Smith , Max Reitz , Paul Durrant , Stefan Hajnoczi , Anthony Perard Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" From: Tim Smith When I/O consists of many small requests, performance is improved by batching them together in a single io_submit() call. When there are relatively few requests, the extra overhead is not worth it. This introduces a check to start batching I/O requests via blk_io_plug()/ blk_io_unplug() in an amount proportional to the number which were already in flight at the time we started reading the ring. Signed-off-by: Tim Smith Re-based and commit comment adjusted. Signed-off-by: Paul Durrant --- Cc: Stefan Hajnoczi Cc: Stefano Stabellini Cc: Anthony Perard Cc: Kevin Wolf Cc: Max Reitz --- hw/block/dataplane/xen-block.c | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c index 80df7da..db17ab5 100644 --- a/hw/block/dataplane/xen-block.c +++ b/hw/block/dataplane/xen-block.c @@ -528,10 +528,18 @@ static int xen_block_get_request(XenBlockDataPlane *dataplane, return 0; } +/* + * Threshold of in-flight requests above which we will start using + * blk_io_plug()/blk_io_unplug() to batch requests. + */ +#define IO_PLUG_THRESHOLD 1 + static void xen_block_handle_requests(XenBlockDataPlane *dataplane) { RING_IDX rc, rp; XenBlockRequest *request; + int inflight_atstart = dataplane->requests_inflight; + int batched = 0; dataplane->more_work = 0; @@ -540,6 +548,18 @@ static void xen_block_handle_requests(XenBlockDataPlane *dataplane) xen_rmb(); /* Ensure we see queued requests up to 'rp'. */ xen_block_send_response_all(dataplane); + /* + * If there was more than IO_PLUG_THRESHOLD requests in flight + * when we got here, this is an indication that there the bottleneck + * is below us, so it's worth beginning to batch up I/O requests + * rather than submitting them immediately. The maximum number + * of requests we're willing to batch is the number already in + * flight, so it can grow up to max_requests when the bottleneck + * is below us. + */ + if (inflight_atstart > IO_PLUG_THRESHOLD) { + blk_io_plug(dataplane->blk); + } while (rc != rp) { /* pull request from ring */ if (RING_REQUEST_CONS_OVERFLOW(&dataplane->rings.common, rc)) { @@ -585,7 +605,22 @@ static void xen_block_handle_requests(XenBlockDataPlane *dataplane) continue; } + if (inflight_atstart > IO_PLUG_THRESHOLD && + batched >= inflight_atstart) { + blk_io_unplug(dataplane->blk); + } xen_block_do_aio(request); + if (inflight_atstart > IO_PLUG_THRESHOLD) { + if (batched >= inflight_atstart) { + blk_io_plug(dataplane->blk); + batched = 0; + } else { + batched++; + } + } + } + if (inflight_atstart > IO_PLUG_THRESHOLD) { + blk_io_unplug(dataplane->blk); } if (dataplane->more_work &&