On Mon, May 01, 2017 at 02:32:43PM -0400, Brian Foster wrote:
> On Fri, Apr 28, 2017 at 12:46:52PM -0700, Darrick J. Wong wrote:
> > Currently, the dir2 leaf block getdents function uses a complex state
> > tracking mechanism to create a shadow copy of the block mappings and
> > then uses the shadow copy to schedule readahead. Since the read and
> > readahead functions are perfectly capable of reading the mappings
> > themselves, we can tear all that out in favor of a simpler function that
> > simply keeps pushing the readahead window further out.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > ---
> > v3: use sliding window to constrain the amount of readahead
> > v2: fix readahead of more than ra_want
> > ---
>
> Thanks for the updates. This looks much more simple and seems true to
> the current readahead behavior. The code also looks fine to me (one bit
> of whitespace damage noted below).
>
> That aside, have you happened to test this against a huge/ugly directory
> to verify it works as expected? Note that I don't think in depth
> performance analysis is required. Verification of any kind of dir that
> is known to benefit from readahead is probably sufficient IMO. Perhaps
> dm-delay with a small enough latency to allow us to measure the effect
> of readahead could help us here.
Yeah, I used xfs/349 to generate a filesystem containing a directory
with a freeindex block, then ls'd the entire directory to see how long
the getdents calls took. The readhead calls were nearly identical with
similar runtimes.
> > fs/xfs/xfs_dir2_readdir.c | 316 ++++++++++++---------------------------------
> > 1 file changed, 82 insertions(+), 234 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_dir2_readdir.c b/fs/xfs/xfs_dir2_readdir.c
> > index 20b7a5c..d05c1ec 100644
> > --- a/fs/xfs/xfs_dir2_readdir.c
> > +++ b/fs/xfs/xfs_dir2_readdir.c
> > @@ -243,214 +243,98 @@ xfs_dir2_block_getdents(
> > return 0;
> > }
> >
> ...
> > /*
> > - * Do we need more readahead?
> > - * Each loop tries to process 1 full dir blk; last may be partial.
> > + * Start readahead for the next bufsize's worth of dir data blocks.
> > + * We may have already issued readahead for some of that range;
> > + * ra_blk tracks the last block we tried to read(ahead).
> > */
> > + ra_want = howmany(bufsize + geo->blksize, (1 << geo->fsblog));
> > + if (*ra_blk >= last_da)
> > + goto out;
> > + else if (*ra_blk == 0)
> > + *ra_blk = map.br_startoff;
> > + next_ra = map.br_startoff + geo->fsbcount;
> > + if (next_ra >= last_da)
> > + goto out_no_ra;
> > + found = xfs_iext_lookup_extent(dp, ifp, next_ra, &idx, &map);
> > + if (!found || map.br_startoff >= last_da)
> > + goto out_no_ra;
> > + xfs_trim_extent(&map, next_ra, last_da - next_ra);
> > +
>
> ^ trailing space.
(I don't see it...?)
--D
>
> Brian
>
> > + /* Start ra for each dir (not fs) block that has a mapping. */
> > blk_start_plug(&plug);
> > - for (mip->ra_index = mip->ra_offset = i = 0;
> > - mip->ra_want > mip->ra_current && i < mip->map_blocks;
> > - i += geo->fsbcount) {
> > - ASSERT(mip->ra_index < mip->map_valid);
> > - /*
> > - * Read-ahead a contiguous directory block.
> > - */
> > - if (i > mip->ra_current &&
> > - (map[mip->ra_index].br_blockcount - mip->ra_offset) >=
> > - geo->fsbcount) {
> > - xfs_dir3_data_readahead(dp,
> > - map[mip->ra_index].br_startoff + mip->ra_offset,
> > - XFS_FSB_TO_DADDR(dp->i_mount,
> > - map[mip->ra_index].br_startblock +
> > - mip->ra_offset));
> > - mip->ra_current = i;
> > - }
> > -
> > - /*
> > - * Read-ahead a non-contiguous directory block. This doesn't
> > - * use our mapping, but this is a very rare case.
> > - */
> > - else if (i > mip->ra_current) {
> > - xfs_dir3_data_readahead(dp,
> > - map[mip->ra_index].br_startoff +
> > - mip->ra_offset, -1);
> > - mip->ra_current = i;
> > - }
> > -
> > - /*
> > - * Advance offset through the mapping table, processing a full
> > - * dir block even if it is fragmented into several extents.
> > - * But stop if we have consumed all valid mappings, even if
> > - * it's not yet a full directory block.
> > - */
> > - for (j = 0;
> > - j < geo->fsbcount && mip->ra_index < mip->map_valid;
> > - j += length ) {
> > - /*
> > - * The rest of this extent but not more than a dir
> > - * block.
> > - */
> > - length = min_t(int, geo->fsbcount - j,
> > - map[mip->ra_index].br_blockcount -
> > - mip->ra_offset);
> > - mip->ra_offset += length;
> > -
> > - /*
> > - * Advance to the next mapping if this one is used up.
> > - */
> > - if (mip->ra_offset == map[mip->ra_index].br_blockcount) {
> > - mip->ra_offset = 0;
> > - mip->ra_index++;
> > + while (ra_want > 0) {
> > + next_ra = roundup((xfs_dablk_t)map.br_startoff, geo->fsbcount);
> > + while (ra_want > 0 &&
> > + next_ra < map.br_startoff + map.br_blockcount) {
> > + if (next_ra >= last_da) {
> > + *ra_blk = last_da;
> > + break;
> > + } else if (next_ra > *ra_blk) {
> > + xfs_dir3_data_readahead(dp, next_ra, -2);
> > + *ra_blk = next_ra;
> > }
> > + ra_want -= geo->fsbcount;
> > + next_ra += geo->fsbcount;
> > + }
> > + found = xfs_iext_get_extent(ifp, ++idx, &map);
> > + if (!found) {
> > + *ra_blk = last_da;
> > + break;
> > }
> > }
> > blk_finish_plug(&plug);
> > @@ -458,6 +342,9 @@ xfs_dir2_leaf_readbuf(
> > out:
> > *bpp = bp;
> > return error;
> > +out_no_ra:
> > + *ra_blk = last_da;
> > + goto out;
> > }
> >
> > /*
> > @@ -475,14 +362,14 @@ xfs_dir2_leaf_getdents(
> > xfs_dir2_data_hdr_t *hdr; /* data block header */
> > xfs_dir2_data_entry_t *dep; /* data entry */
> > xfs_dir2_data_unused_t *dup; /* unused entry */
> > - int error = 0; /* error return value */
> > - int length; /* temporary length value */
> > - int byteoff; /* offset in current block */
> > - xfs_dir2_off_t curoff; /* current overall offset */
> > - xfs_dir2_off_t newoff; /* new curoff after new blk */
> > char *ptr = NULL; /* pointer to current data */
> > - struct xfs_dir2_leaf_map_info *map_info;
> > struct xfs_da_geometry *geo = args->geo;
> > + xfs_dablk_t rablk = 0; /* current readahead block */
> > + xfs_dir2_off_t curoff; /* current overall offset */
> > + int length; /* temporary length value */
> > + int byteoff; /* offset in current block */
> > + int lock_mode;
> > + int error = 0; /* error return value */
> >
> > /*
> > * If the offset is at or past the largest allowed value,
> > @@ -492,30 +379,12 @@ xfs_dir2_leaf_getdents(
> > return 0;
> >
> > /*
> > - * Set up to bmap a number of blocks based on the caller's
> > - * buffer size, the directory block size, and the filesystem
> > - * block size.
> > - */
> > - length = howmany(bufsize + geo->blksize, (1 << geo->fsblog));
> > - map_info = kmem_zalloc(offsetof(struct xfs_dir2_leaf_map_info, map) +
> > - (length * sizeof(struct xfs_bmbt_irec)),
> > - KM_SLEEP | KM_NOFS);
> > - map_info->map_size = length;
> > -
> > - /*
> > * Inside the loop we keep the main offset value as a byte offset
> > * in the directory file.
> > */
> > curoff = xfs_dir2_dataptr_to_byte(ctx->pos);
> >
> > /*
> > - * Force this conversion through db so we truncate the offset
> > - * down to get the start of the data block.
> > - */
> > - map_info->map_off = xfs_dir2_db_to_da(geo,
> > - xfs_dir2_byte_to_db(geo, curoff));
> > -
> > - /*
> > * Loop over directory entries until we reach the end offset.
> > * Get more blocks and readahead as necessary.
> > */
> > @@ -527,38 +396,18 @@ xfs_dir2_leaf_getdents(
> > * current buffer, need to get another one.
> > */
> > if (!bp || ptr >= (char *)bp->b_addr + geo->blksize) {
> > - int lock_mode;
> > - bool trim_map = false;
> > -
> > if (bp) {
> > - xfs_trans_brelse(NULL, bp);
> > + xfs_trans_brelse(args->trans, bp);
> > bp = NULL;
> > - trim_map = true;
> > }
> >
> > lock_mode = xfs_ilock_data_map_shared(dp);
> > - error = xfs_dir2_leaf_readbuf(args, bufsize, map_info,
> > - &curoff, &bp, trim_map);
> > + error = xfs_dir2_leaf_readbuf(args, bufsize, &curoff,
> > + &rablk, &bp);
> > xfs_iunlock(dp, lock_mode);
> > - if (error || !map_info->map_valid)
> > + if (error || !bp)
> > break;
> >
> > - /*
> > - * Having done a read, we need to set a new offset.
> > - */
> > - newoff = xfs_dir2_db_off_to_byte(geo,
> > - map_info->curdb, 0);
> > - /*
> > - * Start of the current block.
> > - */
> > - if (curoff < newoff)
> > - curoff = newoff;
> > - /*
> > - * Make sure we're in the right block.
> > - */
> > - else if (curoff > newoff)
> > - ASSERT(xfs_dir2_byte_to_db(geo, curoff) ==
> > - map_info->curdb);
> > hdr = bp->b_addr;
> > xfs_dir3_data_check(dp, bp);
> > /*
> > @@ -643,7 +492,6 @@ xfs_dir2_leaf_getdents(
> > ctx->pos = XFS_DIR2_MAX_DATAPTR & 0x7fffffff;
> > else
> > ctx->pos = xfs_dir2_byte_to_dataptr(curoff) & 0x7fffffff;
> > - kmem_free(map_info);
> > if (bp)
> > xfs_trans_brelse(NULL, bp);
> > return error;
> > --
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