From 013c3ca184851078b9c04744efd4d47e52c6ecf8 Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Tue, 26 Apr 2011 16:24:18 -0400 Subject: xen/blkback: Stick REQ_SYNC on WRITEs to deal with CFQ I/O scheduler. If one runs a simple fio request with random read/write with a 20%/80% ratio, the numbers are incredibly bad when using the CFQ scheduler. IOmeter | | | | 64K, randrw | NOOP | CFQ | deadline | randrwmix=80 | | | | --------------+-------+------+----------+ blkback |103/27 |32/10 | 102/27 | --------------+-------+------+----------+ QEMU qdisk |103/27 |102/27| 102/27 | The problem as explained by Vivek Goyal was: ".. that difference is that sync vs async requests. In the case of a kernel thread submitting IO, [..] all the WRITES might be being considered as async and will go in a different queue. If you mix those with some READS, they are always sync and will go in differnet queue. In presence of sync queue, CFQ will idle and choke up WRITES in an attempt to improve latencies of READs. In case of AIO [note: this is what QEMU qdisk is doing] , [..] it is direct IO and both READS and WRITES will be considered SYNC and will go in a single queue and no choking of WRITES will take place." The solution is quite simple, tack on REQ_SYNC (which is what the WRITE_ODIRECT macro points to) and the numbers go back up. Suggested-by: Vivek Goyal diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c index ed85ba94..8583b13 100644 --- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -559,7 +559,7 @@ static void dispatch_rw_block_io(struct blkif_st *blkif, operation = READ; break; case BLKIF_OP_WRITE: - operation = WRITE; + operation = WRITE_ODIRECT; break; case BLKIF_OP_WRITE_BARRIER: operation = WRITE_BARRIER; -- cgit v0.10.2