svn commit: r335066 - in head/sys: kern sys

[Warner Losh]

On Wed, Jun 13, 2018 at 1:12 PM, Ravi Pokala <rpokala at freebsd.org> wrote:

> Hi Warner,
>
> I have many questions...
>
> "Car limit"? I'm not sure what you mean by that?
>

So we implement the elevator algorithm, but limit the number of requests
that we can put in each 'car'. Elevators themselves are sometimes call
'cars' for reasons I have no clue about.

So we only put N requests into the sorting algorithm. We put the next
request at the end, unsorted, and that has the side effect of starting over
so the net request after that is sorted against the last one, but not the
first N.

This is to help bound software queuing latency.


> This change starts tracking the total number of BIOs on the queue, but
> doesn't actually do anything with that value.
>

Oh, you're right. We have a sysctl that counts them, I think, in Netflix's
code base (or did, I can't find it in the current sources)... I should
remove that from here. I'm surprised I didn't notice that before.


> When there are more requests batched than the limit, your comment say you
> "start over", and the code says you bioq_insert_tail(). Why does the latter
> imply "start(ing) over"?
>

Because that also sets a new sorting point for inserting sorted.

Warner



> Thanks,
>
> Ravi (rpokala@)
>
> -----Original Message-----
> From: <owner-src-committers at freebsd.org> on behalf of Warner Losh
> <imp at FreeBSD.org>
> Date: 2018-06-13, Wednesday at 12:48
> To: <src-committers at freebsd.org>, <svn-src-all at freebsd.org>, <
> svn-src-head at freebsd.org>
> Subject: svn commit: r335066 - in head/sys: kern sys
>
> Author: imp
> Date: Wed Jun 13 16:48:07 2018
> New Revision: 335066
> URL: https://svnweb.freebsd.org/changeset/base/335066
>
> Log:
>   Implement a 'car limit' for bioq.
>
>   Allow one to implement a 'car limit' for
>   bioq_disksort. debug.bioq_batchsize sets the size of car limit. Every
>   time we queue that many requests, we start over so that we limit the
>   latency for requests when the software queue depths are large. A value
>   of '0', the default, means to revert to the old behavior.
>
>   Sponsored by: Netflix
>
> Modified:
>   head/sys/kern/subr_disk.c
>   head/sys/sys/bio.h
>
> Modified: head/sys/kern/subr_disk.c
> ============================================================
> ==================
> --- head/sys/kern/subr_disk.c   Wed Jun 13 15:58:33 2018        (r335065)
> +++ head/sys/kern/subr_disk.c   Wed Jun 13 16:48:07 2018        (r335066)
> @@ -23,8 +23,13 @@ __FBSDID("$FreeBSD$");
>  #include <sys/bio.h>
>  #include <sys/conf.h>
>  #include <sys/disk.h>
> +#include <sys/sysctl.h>
>  #include <geom/geom_disk.h>
>
> +static int bioq_batchsize = 0;
> +SYSCTL_INT(_debug, OID_AUTO, bioq_batchsize, CTLFLAG_RW,
> +    &bioq_batchsize, 0, "BIOQ batch size");
> +
>  /*-
>   * Disk error is the preface to plaintive error messages
>   * about failing disk transfers.  It prints messages of the form
> @@ -152,6 +157,8 @@ bioq_init(struct bio_queue_head *head)
>         TAILQ_INIT(&head->queue);
>         head->last_offset = 0;
>         head->insert_point = NULL;
> +       head->total = 0;
> +       head->batched = 0;
>  }
>
>  void
> @@ -165,6 +172,7 @@ bioq_remove(struct bio_queue_head *head, struct bio *b
>                 head->insert_point = NULL;
>
>         TAILQ_REMOVE(&head->queue, bp, bio_queue);
> +       head->total--;
>  }
>
>  void
> @@ -183,6 +191,8 @@ bioq_insert_head(struct bio_queue_head *head, struct b
>         if (head->insert_point == NULL)
>                 head->last_offset = bp->bio_offset;
>         TAILQ_INSERT_HEAD(&head->queue, bp, bio_queue);
> +       head->total++;
> +       head->batched = 0;
>  }
>
>  void
> @@ -190,6 +200,7 @@ bioq_insert_tail(struct bio_queue_head *head, struct b
>  {
>
>         TAILQ_INSERT_TAIL(&head->queue, bp, bio_queue);
> +       head->total++;
>         head->insert_point = bp;
>         head->last_offset = bp->bio_offset;
>  }
> @@ -248,6 +259,11 @@ bioq_disksort(struct bio_queue_head *head, struct bio
>                 return;
>         }
>
> +       if (bioq_batchsize > 0 && head->batched > bioq_batchsize) {
> +               bioq_insert_tail(head, bp);
> +               return;
> +       }
> +
>         prev = NULL;
>         key = bioq_bio_key(head, bp);
>         cur = TAILQ_FIRST(&head->queue);
> @@ -266,4 +282,6 @@ bioq_disksort(struct bio_queue_head *head, struct bio
>                 TAILQ_INSERT_HEAD(&head->queue, bp, bio_queue);
>         else
>                 TAILQ_INSERT_AFTER(&head->queue, prev, bp, bio_queue);
> +       head->total++;
> +       head->batched++;
>  }
>
> Modified: head/sys/sys/bio.h
> ============================================================
> ==================
> --- head/sys/sys/bio.h  Wed Jun 13 15:58:33 2018        (r335065)
> +++ head/sys/sys/bio.h  Wed Jun 13 16:48:07 2018        (r335066)
> @@ -138,6 +138,8 @@ struct bio_queue_head {
>         TAILQ_HEAD(bio_queue, bio) queue;
>         off_t last_offset;
>         struct  bio *insert_point;
> +       int total;
> +       int batched;
>  };
>
>  extern struct vm_map *bio_transient_map;
>
>
>
>