linux-kernel - Re: [PATCH 1/4] block: add scalable completion tracking of requests

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Date:   Thu, 3 Nov 2016 08:10:16 -0600
From:   Bart Van Assche <bart.vanassche@...disk.com>
To:     Jens Axboe <axboe@...com>, <axboe@...nel.dk>,
        <linux-kernel@...r.kernel.org>, <linux-block@...r.kernel.org>
CC:     <hch@....de>
Subject: Re: [PATCH 1/4] block: add scalable completion tracking of requests

On 11/01/2016 03:05 PM, Jens Axboe wrote:
> +void blk_stat_init(struct blk_rq_stat *stat)
> +{
> +	__blk_stat_init(stat, ktime_to_ns(ktime_get()));
> +}
> +
> +static bool __blk_stat_is_current(struct blk_rq_stat *stat, s64 now)
> +{
> +	return (now & BLK_STAT_NSEC_MASK) == (stat->time & BLK_STAT_NSEC_MASK);
> +}
> +
> +bool blk_stat_is_current(struct blk_rq_stat *stat)
> +{
> +	return __blk_stat_is_current(stat, ktime_to_ns(ktime_get()));
> +}

Hello Jens,

What is the performance impact of these patches? My experience is that 
introducing ktime_get() in the I/O path of high-performance I/O devices 
measurably slows down I/O. On https://lkml.org/lkml/2016/4/21/107 I read 
that a single ktime_get() call takes about 100 ns.

Bart.