Re: [PATCH 11/14] xfs: dynamically allocate cursors based on maxlevels

Darrick J. Wong
2021-09-22T17:38:24Z

On Tue, Sep 21, 2021 at 09:06:35AM +1000, Dave Chinner wrote:
> On Fri, Sep 17, 2021 at 06:30:10PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > Replace the statically-sized btree cursor zone with dynamically sized
> > allocations so that we can reduce the memory overhead for per-AG bt
> > cursors while handling very tall btrees for rt metadata.
>
> Hmmmmm. We do a *lot* of btree cursor allocation and freeing under
> load. Keeping that in a single slab rather than using heap memory is
> a good idea for stuff like this for many reasons...
>
> I mean, if we are creating a million inodes a second, a rouch
> back-of-the-envelope calculation says we are doing 3-4 million btree
> cursor instantiations a second. That's a lot of short term churn on
> the heap that we don't really need to subject it to. And even a few
> extra instructions in a path called millions of times a second adds
> up to a lot of extra runtime overhead.
>
> So....
>
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> > fs/xfs/libxfs/xfs_btree.c | 40 ++++++++++++++++++++++++++++++++--------
> > fs/xfs/libxfs/xfs_btree.h | 2 --
> > fs/xfs/xfs_super.c | 11 +----------
> > 3 files changed, 33 insertions(+), 20 deletions(-)
> >
> >
> > diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> > index 2486ba22c01d..f9516828a847 100644
> > --- a/fs/xfs/libxfs/xfs_btree.c
> > +++ b/fs/xfs/libxfs/xfs_btree.c
> > @@ -23,11 +23,6 @@
> > #include "xfs_btree_staging.h"
> > #include "xfs_ag.h"
> >
> > -/*
> > - * Cursor allocation zone.
> > - */
> > -kmem_zone_t *xfs_btree_cur_zone;
> > -
> > /*
> > * Btree magic numbers.
> > */
> > @@ -379,7 +374,7 @@ xfs_btree_del_cursor(
> > kmem_free(cur->bc_ops);
> > if (!(cur->bc_flags & XFS_BTREE_LONG_PTRS) && cur->bc_ag.pag)
> > xfs_perag_put(cur->bc_ag.pag);
> > - kmem_cache_free(xfs_btree_cur_zone, cur);
> > + kmem_free(cur);
> > }
> >
> > /*
> > @@ -4927,6 +4922,32 @@ xfs_btree_has_more_records(
> > return block->bb_u.s.bb_rightsib != cpu_to_be32(NULLAGBLOCK);
> > }
> >
> > +/* Compute the maximum allowed height for a given btree type. */
> > +static unsigned int
> > +xfs_btree_maxlevels(
> > + struct xfs_mount *mp,
> > + xfs_btnum_t btnum)
> > +{
> > + switch (btnum) {
> > + case XFS_BTNUM_BNO:
> > + case XFS_BTNUM_CNT:
> > + return mp->m_ag_maxlevels;
> > + case XFS_BTNUM_BMAP:
> > + return max(mp->m_bm_maxlevels[XFS_DATA_FORK],
> > + mp->m_bm_maxlevels[XFS_ATTR_FORK]);
> > + case XFS_BTNUM_INO:
> > + case XFS_BTNUM_FINO:
> > + return M_IGEO(mp)->inobt_maxlevels;
> > + case XFS_BTNUM_RMAP:
> > + return mp->m_rmap_maxlevels;
> > + case XFS_BTNUM_REFC:
> > + return mp->m_refc_maxlevels;
> > + default:
> > + ASSERT(0);
> > + return XFS_BTREE_MAXLEVELS;
> > + }
> > +}
> > +
> > /* Allocate a new btree cursor of the appropriate size. */
> > struct xfs_btree_cur *
> > xfs_btree_alloc_cursor(
> > @@ -4935,13 +4956,16 @@ xfs_btree_alloc_cursor(
> > xfs_btnum_t btnum)
> > {
> > struct xfs_btree_cur *cur;
> > + unsigned int maxlevels = xfs_btree_maxlevels(mp, btnum);
> >
> > - cur = kmem_cache_zalloc(xfs_btree_cur_zone, GFP_NOFS | __GFP_NOFAIL);
> > + ASSERT(maxlevels <= XFS_BTREE_MAXLEVELS);
> > +
> > + cur = kmem_zalloc(xfs_btree_cur_sizeof(maxlevels), KM_NOFS);
>
> Instead of multiple dynamic runtime calculations to determine the
> size to allocate from the heap, which then has to select a slab
> based on size, why don't we just pre-calculate the max size of
> the cursor at XFS module init and use that for the btree cursor slab
> size?

As part of developing the realtime rmapbt and reflink btrees, I computed
the maximum theoretical btree height for a maximally sized realtime
volume. For a realtime volume with 2^52 blocks and a 1k block size, I
estimate that you'd need a 11-level rtrefcount btree cursor. The rtrmap
btree cursor would have to be 28 levels high. Using 4k blocks instead
of 1k blocks, it's not so bad -- 8 for rtrefcount and 17 for rtrmap.

I don't recall exactly what Chandan said the maximum bmbt height would
need to be to support really large data fork mapping structures, but
based on my worst case estimate of 2^54 single-block mappings and a 1k
blocksize, you'd need a 12-level bmbt cursor. For 4k blocks, you'd need
only 8 levels.

The current XFS_BTREE_MAXLEVELS is 9, which just so happens to fit in
248 bytes. I will rework this patch to make xfs_btree_cur_zone supply
256-byte cursors, and the btree code will continue using the zone if 256
bytes is enough space for the cursor.

If we decide later on that we need a zone for larger cursors, I think
the next logical size up (512 bytes) will fit 25 levels, but let's wait
to get there first.

--D

> The memory overhead of the cursor isn't an issue because we've been
> maximally sizing it since forever, and the whole point of a slab
> cache is to minimise allocation overhead of frequently allocated
> objects. It seems to me that we really want to retain these
> properties of the cursor allocator, not give them up just as we're
> in the process of making other modifications that will hit the path
> more frequently than it's ever been hit before...
>
> I like all the dynamic sized guards that this series places in the
> cursor, but I don't think we want to change the way we allocate the
> cursors just to support that.
>
> FWIW, an example of avoidable runtime calculation overhead of
> constants is xlog_calc_unit_res(). These values are actually
> constant for a given transaction reservation, but at 1.6 million
> transactions a second it shows up at #20 on the flat profile of
> functions using the most CPU:
>
> 0.71% [kernel] [k] xlog_calc_unit_res
>
> 0.71% of 32 CPUs for 1.6 million calculations a second of the same
> constants is a non-trivial amount of CPU time to spend doing
> unnecessary repeated calculations.
>
> Even though the btree cursor constant calculations are simpler than
> the log res calculations, they are more frequent. Hence on general
> principles of efficiency, I don't think we want to be replacing high
> frequency, low overhead slab/zone based allocations with heap
> allocations that require repeated constant calculations and
> size->slab redirection....
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com