On Wed, Sep 05, 2018 at 06:19:30PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> When observing the behaviour of an 8EB mkfs execution, I noticed
> that a phase where there are a massive number of read/modify/write
> cycles occurring. I didn't wait for it to complete - it was obvious
> that it was after all the AG headers had been written. That left the
> AGFL initialisation as the likely cause.
>
> When all the AG headers don't fit in the libxfs buffer cache, the
> AGFL init requires re-reading the AGF, the AGFL, the free space tree
> root blocks and the rmap tree root block. They all then get
> modified and written back out. 10 IOs per AG. When you have 8
> million AGs, that's a lot of extra IO.
>
> Change the initialisation algorithm to initialise the AGFL
> immediately after initialising the rest of the headers and
> calculating the minimum AGFL size for that AG. This means the
> modifications will all hit the buffer cache and this will remove the
> IO penalty.
>
> The "worst_freelist" size calculation doesn't change from AG to AG -
> it's based on the physical configuration of the AG, and all AGs have
> the same configuration. hence we only need to calculate this once,
> not for every AG. That allows us to initialise the AGFL immediately
> after the rest of the AG has been initialised rather than in a
> separate pass.
>
> TIme to make a filesystem from scratch, using a zeroed device so the
> force overwrite algorithms are not triggered and -K to avoid
> discards:
>
> FS size 10PB 100PB 1EB
> current mkfs 26.9s 214.8s 2484s
> patched 11.3s 70.3s 709s
>
> In both cases, the IO profile looks identical for the initial AG
> header writeout loop. The difference is that the old code then
> does the RMW loop to init the AGFL, and that runs at about half the
> speed. Hence runtime of the new code is reduce by around 65-70%
> simply by avoiding all that IO.
>
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
This one seems like it stands alone as a nice fixup. Were you planning
to send this independently as a non-rfc patch?
A couple nits...
> mkfs/xfs_mkfs.c | 40 +++++++++++++++++++++++++---------------
> 1 file changed, 25 insertions(+), 15 deletions(-)
>
> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> index c153592c705e..d70fbdb6b15a 100644
> --- a/mkfs/xfs_mkfs.c
> +++ b/mkfs/xfs_mkfs.c
> @@ -3374,7 +3374,7 @@ initialise_ag_headers(
> struct xfs_mount *mp,
> struct xfs_sb *sbp,
> xfs_agnumber_t agno,
> - int *worst_freelist)
> + int *freelist_size)
> {
> struct xfs_perag *pag = libxfs_perag_get(mp, agno);
> struct xfs_agfl *agfl;
> @@ -3453,8 +3453,22 @@ initialise_ag_headers(
> agf->agf_longest = cpu_to_be32(agsize -
> XFS_FSB_TO_AGBNO(mp, cfg->logstart) - cfg->logblocks);
> }
> - if (libxfs_alloc_min_freelist(mp, pag) > *worst_freelist)
> - *worst_freelist = libxfs_alloc_min_freelist(mp, pag);
> +
> + /*
> + * The AGFL size is the same for all AGs because all AGs have the same
> + * layout. If this AG sameness ever changes in the future, we'll need to
> + * revisit how we initialise the AGFLs.
> + */
This is not necessarily the case if the last AG is not full size, right?
I think the comment could point that out (and/or that this works so long
as we don't process the last AG first).
BTW, libxfs_alloc_min_freelist() uses the ->pagf_levels values for the
bno, cnt and rmap btrees to establish the freelist size, and I don't see
where we've assigned ->pagf_levels[XFS_BTNUM_RMAPi] anywhere.
Brian
> + if (*freelist_size == 0)
> + *freelist_size = libxfs_alloc_min_freelist(mp, pag);
> + else if (*freelist_size < libxfs_alloc_min_freelist(mp, pag)) {
> + fprintf(stderr,
> +_("%s: Abort! Freelist size (%u) for AG %u not constant (%u)!\n"),
> + progname, libxfs_alloc_min_freelist(mp, pag),
> + agno, *freelist_size);
> + exit(1);
> + }
> +
> libxfs_writebuf(buf, LIBXFS_EXIT_ON_FAILURE);
>
> /*
> @@ -3724,14 +3738,14 @@ static void
> initialise_ag_freespace(
> struct xfs_mount *mp,
> xfs_agnumber_t agno,
> - int worst_freelist)
> + int freelist_size)
> {
> struct xfs_alloc_arg args;
> struct xfs_trans *tp;
> struct xfs_trans_res tres = {0};
> int c;
>
> - c = libxfs_trans_alloc(mp, &tres, worst_freelist, 0, 0, &tp);
> + c = libxfs_trans_alloc(mp, &tres, freelist_size, 0, 0, &tp);
> if (c)
> res_failed(c);
>
> @@ -3797,7 +3811,7 @@ main(
> int quiet = 0;
> char *protofile = NULL;
> char *protostring = NULL;
> - int worst_freelist = 0;
> + int freelist_size = 0;
>
> struct libxfs_xinit xi = {
> .isdirect = LIBXFS_DIRECT,
> @@ -4025,16 +4039,12 @@ main(
> }
>
> /*
> - * Initialise all the static on disk metadata.
> + * Initialise all the AG headers on disk.
> */
> - for (agno = 0; agno < cfg.agcount; agno++)
> - initialise_ag_headers(&cfg, mp, sbp, agno, &worst_freelist);
> -
> - /*
> - * Initialise the freespace freelists (i.e. AGFLs) in each AG.
> - */
> - for (agno = 0; agno < cfg.agcount; agno++)
> - initialise_ag_freespace(mp, agno, worst_freelist);
> + for (agno = 0; agno < cfg.agcount; agno++) {
> + initialise_ag_headers(&cfg, mp, sbp, agno, &freelist_size);
> + initialise_ag_freespace(mp, agno, freelist_size);
> + }
>
> /*
> * Allocate the root inode and anything else in the proto file.
> --
> 2.17.0
>
