@@ -1073,6 +1073,7 @@ void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
if (tsk->plug)
return;
+ plug->cur_ktime = 0;
plug->mq_list = NULL;
plug->cached_rq = NULL;
plug->nr_ios = min_t(unsigned short, nr_ios, BLK_MAX_REQUEST_COUNT);
@@ -943,6 +943,7 @@ struct blk_plug {
/* if ios_left is > 1, we can batch tag/rq allocations */
struct request *cached_rq;
+ u64 cur_ktime;
unsigned short nr_ios;
unsigned short rq_count;
@@ -978,7 +979,19 @@ long nr_blockdev_pages(void);
static inline u64 blk_time_get_ns(void)
{
- return ktime_get_ns();
+ struct blk_plug *plug = current->plug;
+
+ if (!plug)
+ return ktime_get_ns();
+
+ /*
+ * 0 could very well be a valid time, but rather than flag "this is
+ * a valid timestamp" separately, just accept that we'll do an extra
+ * ktime_get_ns() if we just happen to get 0 as the current time.
+ */
+ if (!plug->cur_ktime)
+ plug->cur_ktime = ktime_get_ns();
+ return plug->cur_ktime;
}
static inline ktime_t blk_time_get(void)
Querying the current time is the most costly thing we do in the block layer per IO, and depending on kernel config settings, we may do it many times per IO. None of the callers actually need nsec granularity. Take advantage of that by caching the current time in the plug, with the assumption here being that any time checking will be temporally close enough that the slight loss of precision doesn't matter. If the block plug gets flushed, eg on preempt or schedule out, then we invalidate the cached clock. On a basic peak IOPS test case with iostats enabled, this changes the performance from: IOPS=108.41M, BW=52.93GiB/s, IOS/call=31/31 IOPS=108.43M, BW=52.94GiB/s, IOS/call=32/32 IOPS=108.29M, BW=52.88GiB/s, IOS/call=31/32 IOPS=108.35M, BW=52.91GiB/s, IOS/call=32/32 IOPS=108.42M, BW=52.94GiB/s, IOS/call=31/31 IOPS=108.40M, BW=52.93GiB/s, IOS/call=32/32 IOPS=108.31M, BW=52.89GiB/s, IOS/call=32/31 to IOPS=118.79M, BW=58.00GiB/s, IOS/call=31/32 IOPS=118.62M, BW=57.92GiB/s, IOS/call=31/31 IOPS=118.80M, BW=58.01GiB/s, IOS/call=32/31 IOPS=118.78M, BW=58.00GiB/s, IOS/call=32/32 IOPS=118.69M, BW=57.95GiB/s, IOS/call=32/31 IOPS=118.62M, BW=57.92GiB/s, IOS/call=32/31 IOPS=118.63M, BW=57.92GiB/s, IOS/call=31/32 which is more than a 9% improvement in performance. Looking at perf diff, we can see a huge reduction in time overhead: 10.55% -9.88% [kernel.vmlinux] [k] read_tsc 1.31% -1.22% [kernel.vmlinux] [k] ktime_get Note that since this relies on blk_plug for the caching, it's only applicable to the issue side. But this is where most of the time calls happen anyway. On the completion side, cached time stamping is done with struct io_comp patch, as long as the driver supports it. It's also worth noting that the above testing doesn't enable any of the higher cost CPU items on the block layer side, like wbt, cgroups, iocost, etc, which all would add additional time querying and hence overhead. IOW, results would likely look even better in comparison with those enabled, as distros would do. Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: Jens Axboe <axboe@kernel.dk> --- block/blk-core.c | 1 + include/linux/blkdev.h | 15 ++++++++++++++- 2 files changed, 15 insertions(+), 1 deletion(-)