diff mbox series

[v3] xfs: cache minimum realtime summary level

Message ID 95c54025014c6d5d3944924b52637cdc2b50cf4e.1542137269.git.osandov@fb.com (mailing list archive)
State Accepted, archived
Headers show
Series [v3] xfs: cache minimum realtime summary level | expand

Commit Message

Omar Sandoval Nov. 13, 2018, 7:28 p.m. UTC
From: Omar Sandoval <osandov@fb.com>

The realtime summary is a two-dimensional array on disk, effectively:

u32 rsum[log2(number of realtime extents) + 1][number of blocks in the bitmap]

rsum[log][bbno] is the number of extents of size 2**log which start in
bitmap block bbno.

xfs_rtallocate_extent_near() uses xfs_rtany_summary() to check whether
rsum[log][bbno] != 0 for any log level. However, the summary array is
stored in row-major order (i.e., like an array in C), so all of these
entries are not adjacent, but rather spread across the entire summary
file. In the worst case (a full bitmap block), xfs_rtany_summary() has
to check every level.

This means that on a moderately-used realtime device, an allocation will
waste a lot of time finding, reading, and releasing buffers for the
realtime summary. In particular, one of our storage services (which runs
on servers with 8 very slow CPUs and 15 8 TB XFS realtime filesystems)
spends almost 5% of its CPU cycles in xfs_rtbuf_get() and
xfs_trans_brelse() called from xfs_rtany_summary().

One solution would be to also store the summary with the dimensions
swapped. However, this would require a disk format change to a very old
component of XFS.

Instead, we can cache the minimum size which contains any extents. We do
so lazily; rather than guaranteeing that the cache contains the precise
minimum, it always contains a loose lower bound which we tighten when we
read or update a summary block. This only uses a few kilobytes of memory
and is already serialized via the realtime bitmap and summary inode
locks, so the cost is minimal. With this change, the same workload only
spends 0.2% of its CPU cycles in the realtime allocator.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
Based on Linus' master branch.

Changes from v2:
- Allow the cache allocation to fail, in which case we just don't use it

Changes from v1:
- Clarify comment in xfs_rtmount_inodes().
- Use kmem_* instead of kvmalloc/kvfree

 fs/xfs/libxfs/xfs_rtbitmap.c |  6 ++++++
 fs/xfs/xfs_mount.h           |  7 +++++++
 fs/xfs/xfs_rtalloc.c         | 25 +++++++++++++++++++++----
 3 files changed, 34 insertions(+), 4 deletions(-)

Comments

Omar Sandoval Nov. 26, 2018, 5:33 p.m. UTC | #1
On Tue, Nov 13, 2018 at 11:28:59AM -0800, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> The realtime summary is a two-dimensional array on disk, effectively:
> 
> u32 rsum[log2(number of realtime extents) + 1][number of blocks in the bitmap]
> 
> rsum[log][bbno] is the number of extents of size 2**log which start in
> bitmap block bbno.
> 
> xfs_rtallocate_extent_near() uses xfs_rtany_summary() to check whether
> rsum[log][bbno] != 0 for any log level. However, the summary array is
> stored in row-major order (i.e., like an array in C), so all of these
> entries are not adjacent, but rather spread across the entire summary
> file. In the worst case (a full bitmap block), xfs_rtany_summary() has
> to check every level.
> 
> This means that on a moderately-used realtime device, an allocation will
> waste a lot of time finding, reading, and releasing buffers for the
> realtime summary. In particular, one of our storage services (which runs
> on servers with 8 very slow CPUs and 15 8 TB XFS realtime filesystems)
> spends almost 5% of its CPU cycles in xfs_rtbuf_get() and
> xfs_trans_brelse() called from xfs_rtany_summary().
> 
> One solution would be to also store the summary with the dimensions
> swapped. However, this would require a disk format change to a very old
> component of XFS.
> 
> Instead, we can cache the minimum size which contains any extents. We do
> so lazily; rather than guaranteeing that the cache contains the precise
> minimum, it always contains a loose lower bound which we tighten when we
> read or update a summary block. This only uses a few kilobytes of memory
> and is already serialized via the realtime bitmap and summary inode
> locks, so the cost is minimal. With this change, the same workload only
> spends 0.2% of its CPU cycles in the realtime allocator.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> Based on Linus' master branch.
> 
> Changes from v2:
> - Allow the cache allocation to fail, in which case we just don't use it
> 
> Changes from v1:
> - Clarify comment in xfs_rtmount_inodes().
> - Use kmem_* instead of kvmalloc/kvfree
> 
>  fs/xfs/libxfs/xfs_rtbitmap.c |  6 ++++++
>  fs/xfs/xfs_mount.h           |  7 +++++++
>  fs/xfs/xfs_rtalloc.c         | 25 +++++++++++++++++++++----
>  3 files changed, 34 insertions(+), 4 deletions(-)

Ping.
Darrick J. Wong Nov. 26, 2018, 10:57 p.m. UTC | #2
On Tue, Nov 13, 2018 at 11:28:59AM -0800, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> The realtime summary is a two-dimensional array on disk, effectively:
> 
> u32 rsum[log2(number of realtime extents) + 1][number of blocks in the bitmap]
> 
> rsum[log][bbno] is the number of extents of size 2**log which start in
> bitmap block bbno.
> 
> xfs_rtallocate_extent_near() uses xfs_rtany_summary() to check whether
> rsum[log][bbno] != 0 for any log level. However, the summary array is
> stored in row-major order (i.e., like an array in C), so all of these
> entries are not adjacent, but rather spread across the entire summary
> file. In the worst case (a full bitmap block), xfs_rtany_summary() has
> to check every level.
> 
> This means that on a moderately-used realtime device, an allocation will
> waste a lot of time finding, reading, and releasing buffers for the
> realtime summary. In particular, one of our storage services (which runs
> on servers with 8 very slow CPUs and 15 8 TB XFS realtime filesystems)
> spends almost 5% of its CPU cycles in xfs_rtbuf_get() and
> xfs_trans_brelse() called from xfs_rtany_summary().
> 
> One solution would be to also store the summary with the dimensions
> swapped. However, this would require a disk format change to a very old
> component of XFS.
> 
> Instead, we can cache the minimum size which contains any extents. We do
> so lazily; rather than guaranteeing that the cache contains the precise
> minimum, it always contains a loose lower bound which we tighten when we
> read or update a summary block. This only uses a few kilobytes of memory
> and is already serialized via the realtime bitmap and summary inode
> locks, so the cost is minimal. With this change, the same workload only
> spends 0.2% of its CPU cycles in the realtime allocator.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Looks good, will put this in my tree for 4.21/5.0.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> Based on Linus' master branch.
> 
> Changes from v2:
> - Allow the cache allocation to fail, in which case we just don't use it
> 
> Changes from v1:
> - Clarify comment in xfs_rtmount_inodes().
> - Use kmem_* instead of kvmalloc/kvfree
> 
>  fs/xfs/libxfs/xfs_rtbitmap.c |  6 ++++++
>  fs/xfs/xfs_mount.h           |  7 +++++++
>  fs/xfs/xfs_rtalloc.c         | 25 +++++++++++++++++++++----
>  3 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_rtbitmap.c b/fs/xfs/libxfs/xfs_rtbitmap.c
> index b228c821bae6..eaaff67e9626 100644
> --- a/fs/xfs/libxfs/xfs_rtbitmap.c
> +++ b/fs/xfs/libxfs/xfs_rtbitmap.c
> @@ -505,6 +505,12 @@ xfs_rtmodify_summary_int(
>  		uint first = (uint)((char *)sp - (char *)bp->b_addr);
>  
>  		*sp += delta;
> +		if (mp->m_rsum_cache) {
> +			if (*sp == 0 && log == mp->m_rsum_cache[bbno])
> +				mp->m_rsum_cache[bbno]++;
> +			if (*sp != 0 && log < mp->m_rsum_cache[bbno])
> +				mp->m_rsum_cache[bbno] = log;
> +		}
>  		xfs_trans_log_buf(tp, bp, first, first + sizeof(*sp) - 1);
>  	}
>  	if (sum)
> diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> index 7964513c3128..39f04aca8c3a 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -89,6 +89,13 @@ typedef struct xfs_mount {
>  	int			m_logbsize;	/* size of each log buffer */
>  	uint			m_rsumlevels;	/* rt summary levels */
>  	uint			m_rsumsize;	/* size of rt summary, bytes */
> +	/*
> +	 * Optional cache of rt summary level per bitmap block with the
> +	 * invariant that m_rsum_cache[bbno] <= the minimum i for which
> +	 * rsum[i][bbno] != 0. Reads and writes are serialized by the rsumip
> +	 * inode lock.
> +	 */
> +	uint8_t			*m_rsum_cache;
>  	struct xfs_inode	*m_rbmip;	/* pointer to bitmap inode */
>  	struct xfs_inode	*m_rsumip;	/* pointer to summary inode */
>  	struct xfs_inode	*m_rootip;	/* pointer to root directory */
> diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
> index 926ed314ffba..aefd63d46397 100644
> --- a/fs/xfs/xfs_rtalloc.c
> +++ b/fs/xfs/xfs_rtalloc.c
> @@ -64,8 +64,12 @@ xfs_rtany_summary(
>  	int		log;		/* loop counter, log2 of ext. size */
>  	xfs_suminfo_t	sum;		/* summary data */
>  
> +	/* There are no extents at levels < m_rsum_cache[bbno]. */
> +	if (mp->m_rsum_cache && low < mp->m_rsum_cache[bbno])
> +		low = mp->m_rsum_cache[bbno];
> +
>  	/*
> -	 * Loop over logs of extent sizes.  Order is irrelevant.
> +	 * Loop over logs of extent sizes.
>  	 */
>  	for (log = low; log <= high; log++) {
>  		/*
> @@ -80,13 +84,17 @@ xfs_rtany_summary(
>  		 */
>  		if (sum) {
>  			*stat = 1;
> -			return 0;
> +			goto out;
>  		}
>  	}
>  	/*
>  	 * Found nothing, return failure.
>  	 */
>  	*stat = 0;
> +out:
> +	/* There were no extents at levels < log. */
> +	if (mp->m_rsum_cache && log > mp->m_rsum_cache[bbno])
> +		mp->m_rsum_cache[bbno] = log;
>  	return 0;
>  }
>  
> @@ -1187,8 +1195,8 @@ xfs_rtmount_init(
>  }
>  
>  /*
> - * Get the bitmap and summary inodes into the mount structure
> - * at mount time.
> + * Get the bitmap and summary inodes and the summary cache into the mount
> + * structure at mount time.
>   */
>  int					/* error */
>  xfs_rtmount_inodes(
> @@ -1211,6 +1219,14 @@ xfs_rtmount_inodes(
>  		return error;
>  	}
>  	ASSERT(mp->m_rsumip != NULL);
> +	/*
> +	 * The rsum cache is initialized to all zeroes, which is trivially a
> +	 * lower bound on the minimum level with any free extents. We can
> +	 * continue without the cache if it couldn't be allocated.
> +	 */
> +	mp->m_rsum_cache = kmem_zalloc_large(sbp->sb_rbmblocks, KM_SLEEP);
> +	if (!mp->m_rsum_cache)
> +		xfs_warn(mp, "could not allocate realtime summary cache");
>  	return 0;
>  }
>  
> @@ -1218,6 +1234,7 @@ void
>  xfs_rtunmount_inodes(
>  	struct xfs_mount	*mp)
>  {
> +	kmem_free(mp->m_rsum_cache);
>  	if (mp->m_rbmip)
>  		xfs_irele(mp->m_rbmip);
>  	if (mp->m_rsumip)
> -- 
> 2.19.1
>
diff mbox series

Patch

diff --git a/fs/xfs/libxfs/xfs_rtbitmap.c b/fs/xfs/libxfs/xfs_rtbitmap.c
index b228c821bae6..eaaff67e9626 100644
--- a/fs/xfs/libxfs/xfs_rtbitmap.c
+++ b/fs/xfs/libxfs/xfs_rtbitmap.c
@@ -505,6 +505,12 @@  xfs_rtmodify_summary_int(
 		uint first = (uint)((char *)sp - (char *)bp->b_addr);
 
 		*sp += delta;
+		if (mp->m_rsum_cache) {
+			if (*sp == 0 && log == mp->m_rsum_cache[bbno])
+				mp->m_rsum_cache[bbno]++;
+			if (*sp != 0 && log < mp->m_rsum_cache[bbno])
+				mp->m_rsum_cache[bbno] = log;
+		}
 		xfs_trans_log_buf(tp, bp, first, first + sizeof(*sp) - 1);
 	}
 	if (sum)
diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 7964513c3128..39f04aca8c3a 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -89,6 +89,13 @@  typedef struct xfs_mount {
 	int			m_logbsize;	/* size of each log buffer */
 	uint			m_rsumlevels;	/* rt summary levels */
 	uint			m_rsumsize;	/* size of rt summary, bytes */
+	/*
+	 * Optional cache of rt summary level per bitmap block with the
+	 * invariant that m_rsum_cache[bbno] <= the minimum i for which
+	 * rsum[i][bbno] != 0. Reads and writes are serialized by the rsumip
+	 * inode lock.
+	 */
+	uint8_t			*m_rsum_cache;
 	struct xfs_inode	*m_rbmip;	/* pointer to bitmap inode */
 	struct xfs_inode	*m_rsumip;	/* pointer to summary inode */
 	struct xfs_inode	*m_rootip;	/* pointer to root directory */
diff --git a/fs/xfs/xfs_rtalloc.c b/fs/xfs/xfs_rtalloc.c
index 926ed314ffba..aefd63d46397 100644
--- a/fs/xfs/xfs_rtalloc.c
+++ b/fs/xfs/xfs_rtalloc.c
@@ -64,8 +64,12 @@  xfs_rtany_summary(
 	int		log;		/* loop counter, log2 of ext. size */
 	xfs_suminfo_t	sum;		/* summary data */
 
+	/* There are no extents at levels < m_rsum_cache[bbno]. */
+	if (mp->m_rsum_cache && low < mp->m_rsum_cache[bbno])
+		low = mp->m_rsum_cache[bbno];
+
 	/*
-	 * Loop over logs of extent sizes.  Order is irrelevant.
+	 * Loop over logs of extent sizes.
 	 */
 	for (log = low; log <= high; log++) {
 		/*
@@ -80,13 +84,17 @@  xfs_rtany_summary(
 		 */
 		if (sum) {
 			*stat = 1;
-			return 0;
+			goto out;
 		}
 	}
 	/*
 	 * Found nothing, return failure.
 	 */
 	*stat = 0;
+out:
+	/* There were no extents at levels < log. */
+	if (mp->m_rsum_cache && log > mp->m_rsum_cache[bbno])
+		mp->m_rsum_cache[bbno] = log;
 	return 0;
 }
 
@@ -1187,8 +1195,8 @@  xfs_rtmount_init(
 }
 
 /*
- * Get the bitmap and summary inodes into the mount structure
- * at mount time.
+ * Get the bitmap and summary inodes and the summary cache into the mount
+ * structure at mount time.
  */
 int					/* error */
 xfs_rtmount_inodes(
@@ -1211,6 +1219,14 @@  xfs_rtmount_inodes(
 		return error;
 	}
 	ASSERT(mp->m_rsumip != NULL);
+	/*
+	 * The rsum cache is initialized to all zeroes, which is trivially a
+	 * lower bound on the minimum level with any free extents. We can
+	 * continue without the cache if it couldn't be allocated.
+	 */
+	mp->m_rsum_cache = kmem_zalloc_large(sbp->sb_rbmblocks, KM_SLEEP);
+	if (!mp->m_rsum_cache)
+		xfs_warn(mp, "could not allocate realtime summary cache");
 	return 0;
 }
 
@@ -1218,6 +1234,7 @@  void
 xfs_rtunmount_inodes(
 	struct xfs_mount	*mp)
 {
+	kmem_free(mp->m_rsum_cache);
 	if (mp->m_rbmip)
 		xfs_irele(mp->m_rbmip);
 	if (mp->m_rsumip)