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[PATCH] [RFC v2] xfs: byte range buffer dirty region tracking

To: xfs@xxxxxxxxxxx
Subject: [PATCH] [RFC v2] xfs: byte range buffer dirty region tracking
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 29 May 2015 12:16:17 +1000
Delivered-to: xfs@xxxxxxxxxxx
From: Dave Chinner <dchinner@xxxxxxxxxx>

The biggest problem with large directory block sizes is the CPU
overhead in maintaining the buffer log item direty region bitmap.
The bit manipulations and buffer region mapping calls are right at
the top of the profiles when running tests on 64k directory buffers:

+  16.65%  [kernel]  [k] memcpy
+  11.99%  [kernel]  [k] xfs_next_bit
+   5.87%  [kernel]  [k] xfs_buf_item_format
+   5.85%  [kernel]  [k] xfs_buf_item_size_segment.isra.4
+   5.72%  [kernel]  [k] xfs_buf_offset

The memcpy is the copying of the dirty regions into the log vec
array, but almost twice as much CPU time is spent working out what
needs to be copied and where it needs to be copied from. As a
result, on a production kernel creating 100,000 entries in a 64k
directory runs at about 9,000 files/s while on a 4k directory block
size it runs at 19,000 files/s - about half the speed.

Switching this to just track the first and last modified bytes in
the block and only converting that to a dirty bitmap in the buffer
log item at format time, however, gets rid of most of this dirty
bitmap overhead without increasing memcpy time  at all. the result
is that peformance on a 64k directory block size increases to
roughly 16,000 files/s with memcpy() overhead only slightly
increasing.

This code works - it passes all my xfstests, stress and benchmark
workloads, and has for a couple of months now. Hence it's time to
ask the questions of how best to approach the region count vs log
bandwidth tradeoff....

Discussion:

I think that we will eventually need to track multiple regions - 3
is probably sufficient - because the nature of directory operations
are that just about every operation modifies a header in the buffer,
a tail section in the buffer and then some number of bytes/regions
in the middle of the buffer.

Hence if we just track a single region, it will almost always cover
the entire directory buffer - if we only modify a single entry in
the buffer, then that's a fairly large cost in terms of log space
and CPU overhead for random individual operations. If we decide that
we are going to use a single range, then for directories we might be
better to just use the dirty flag and log the entire buffer every
time.

We also have to consider non-directory buffer modification patterns.
freespace, inode and extent btrees are the other major types of
buffers that get logged, but they also have modification patterns
that lend themselves well to a small number of ranges for dirty
tracking. That is, each btree block is kept compact, so when we
insert or remove a record or pointer we shift then higher
records/ptrs up or down as a block, and then log the lot of them.
And they also often have a header that is dirtied with each
insert/delete, so typically there are usually only one or two dirty
ranges in a btree block.

The only metadata type that really seems to benefit from fine
grained dirty range logging is the inode buffers. Specifically, for
v4 superblocks the create transaction only dirties the regions of
the inode core, so for 256 byte inodes only dirties every alternate
bitmap segement.  Dirty range tracking will double the required log
bandwidth of inode buffers during create (roughly 25% increase on a
4k directory block size filesystem). There isn't any performance
differential noticable on typical systems because the log is far
from being bandwidth bound.

For v5 filesystems, this isn't an issue because the initialised
inode buffers are XFS_BLI_ORDERED buffers and so their contents
aren't logged.

The same problem happens with unlinks due to the unlinked list being
logged via the inode buffer. Again this results in an increase
in log bandwidth on both v4 and v5 filesystems, but there isn't any
performance differential that occurs because, again, the log isn't
bandwidth bound. As it is, there is an existing plan of improvement
to the unlinked list logging (moving the unlinked list logging into
the inode core transaction) and hence that will avoid any extra
overhead here as well.

Overall, I think this is a no-brainer given the performance
improvements we get on large directory block size filesystems. There
are some downsides, but looking at the medium-term development plans
we will mitigate the impact of most of those downsides.

Comments?

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/xfs_buf.c      |   2 +
 fs/xfs/xfs_buf_item.c | 294 +++++++++++++++-----------------------------------
 fs/xfs/xfs_buf_item.h |  19 ++++
 3 files changed, 110 insertions(+), 205 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 1790b00..bbe4e9e 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -1457,6 +1457,8 @@ xfs_buf_iomove(
                page = bp->b_pages[page_index];
                csize = min_t(size_t, PAGE_SIZE - page_offset,
                                      BBTOB(bp->b_io_length) - boff);
+               if (boff + csize > bend)
+                       csize = bend - boff;
 
                ASSERT((csize + page_offset) <= PAGE_SIZE);
 
diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
index 092d652..1816334 100644
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -65,50 +65,12 @@ xfs_buf_item_size_segment(
        int                     *nvecs,
        int                     *nbytes)
 {
-       struct xfs_buf          *bp = bip->bli_buf;
-       int                     next_bit;
-       int                     last_bit;
-
-       last_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size, 0);
-       if (last_bit == -1)
-               return;
-
        /*
         * initial count for a dirty buffer is 2 vectors - the format structure
-        * and the first dirty region.
+        * and the dirty region. Dirty region is accounted for separately.
         */
        *nvecs += 2;
-       *nbytes += xfs_buf_log_format_size(blfp) + XFS_BLF_CHUNK;
-
-       while (last_bit != -1) {
-               /*
-                * This takes the bit number to start looking from and
-                * returns the next set bit from there.  It returns -1
-                * if there are no more bits set or the start bit is
-                * beyond the end of the bitmap.
-                */
-               next_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size,
-                                       last_bit + 1);
-               /*
-                * If we run out of bits, leave the loop,
-                * else if we find a new set of bits bump the number of vecs,
-                * else keep scanning the current set of bits.
-                */
-               if (next_bit == -1) {
-                       break;
-               } else if (next_bit != last_bit + 1) {
-                       last_bit = next_bit;
-                       (*nvecs)++;
-               } else if (xfs_buf_offset(bp, next_bit * XFS_BLF_CHUNK) !=
-                          (xfs_buf_offset(bp, last_bit * XFS_BLF_CHUNK) +
-                           XFS_BLF_CHUNK)) {
-                       last_bit = next_bit;
-                       (*nvecs)++;
-               } else {
-                       last_bit++;
-               }
-               *nbytes += XFS_BLF_CHUNK;
-       }
+       *nbytes += xfs_buf_log_format_size(blfp);
 }
 
 /*
@@ -135,6 +97,8 @@ xfs_buf_item_size(
        int                     *nbytes)
 {
        struct xfs_buf_log_item *bip = BUF_ITEM(lip);
+       struct xfs_buf  *bp = bip->bli_buf;
+       uint                    offset = 0;
        int                     i;
 
        ASSERT(atomic_read(&bip->bli_refcount) > 0);
@@ -154,6 +118,7 @@ xfs_buf_item_size(
        }
 
        ASSERT(bip->bli_flags & XFS_BLI_LOGGED);
+       ASSERT(bip->bli_flags & XFS_BLI_DIRTY);
 
        if (bip->bli_flags & XFS_BLI_ORDERED) {
                /*
@@ -175,10 +140,28 @@ xfs_buf_item_size(
         * count for the extra buf log format structure that will need to be
         * written.
         */
+       ASSERT(bip->bli_range[0].last != 0);
+       if (bip->bli_range[0].last == 0) {
+               /* clean! */
+               ASSERT(bip->bli_range[0].first == 0);
+               return;
+       }
+
        for (i = 0; i < bip->bli_format_count; i++) {
-               xfs_buf_item_size_segment(bip, &bip->bli_formats[i],
-                                         nvecs, nbytes);
+               /*
+                * Only format dirty regions or stale buffers
+                */
+               struct xfs_bli_range *rp = &bip->bli_range[0];
+
+               if ((rp->first <= offset + BBTOB(bp->b_maps[i].bm_len) &&
+                    rp->last >= offset))
+                       xfs_buf_item_size_segment(bip, &bip->bli_formats[i],
+                                                 nvecs, nbytes);
+               offset += BBTOB(bp->b_maps[i].bm_len);
        }
+       *nbytes += bip->bli_range[0].last - bip->bli_range[0].first;
+
+
        trace_xfs_buf_item_size(bip);
 }
 
@@ -191,7 +174,6 @@ xfs_buf_item_copy_iovec(
        int                     first_bit,
        uint                    nbits)
 {
-       offset += first_bit * XFS_BLF_CHUNK;
        xlog_copy_iovec(lv, vecp, XLOG_REG_TYPE_BCHUNK,
                        xfs_buf_offset(bp, offset),
                        nbits * XFS_BLF_CHUNK);
@@ -214,14 +196,18 @@ xfs_buf_item_format_segment(
        struct xfs_buf_log_item *bip,
        struct xfs_log_vec      *lv,
        struct xfs_log_iovec    **vecp,
+       struct xfs_bli_range    *rp,
        uint                    offset,
+       uint                    length,
        struct xfs_buf_log_format *blfp)
 {
        struct xfs_buf  *bp = bip->bli_buf;
+       char            *buf;
        uint            base_size;
+       uint            start;
+       uint            end;
        int             first_bit;
        int             last_bit;
-       int             next_bit;
        uint            nbits;
 
        /* copy the flags across from the base format item */
@@ -233,16 +219,6 @@ xfs_buf_item_format_segment(
         * memory structure.
         */
        base_size = xfs_buf_log_format_size(blfp);
-
-       first_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size, 0);
-       if (!(bip->bli_flags & XFS_BLI_STALE) && first_bit == -1) {
-               /*
-                * If the map is not be dirty in the transaction, mark
-                * the size as zero and do not advance the vector pointer.
-                */
-               return;
-       }
-
        blfp = xlog_copy_iovec(lv, vecp, XLOG_REG_TYPE_BFORMAT, blfp, 
base_size);
        blfp->blf_size = 1;
 
@@ -257,46 +233,40 @@ xfs_buf_item_format_segment(
                return;
        }
 
+       blfp->blf_size++;
 
        /*
-        * Fill in an iovec for each set of contiguous chunks.
+        * Now we need to set the bits in the bitmap and set up the iovecs
+        * appropriately. We know there is a contiguous range in this buffer
+        * than needs to be set, so find the first bit, the last bit, and
+        * go from there.
         */
-       last_bit = first_bit;
-       nbits = 1;
-       for (;;) {
-               /*
-                * This takes the bit number to start looking from and
-                * returns the next set bit from there.  It returns -1
-                * if there are no more bits set or the start bit is
-                * beyond the end of the bitmap.
-                */
-               next_bit = xfs_next_bit(blfp->blf_data_map, blfp->blf_map_size,
-                                       (uint)last_bit + 1);
-               /*
-                * If we run out of bits fill in the last iovec and get out of
-                * the loop.  Else if we start a new set of bits then fill in
-                * the iovec for the series we were looking at and start
-                * counting the bits in the new one.  Else we're still in the
-                * same set of bits so just keep counting and scanning.
-                */
-               if (next_bit == -1) {
-                       xfs_buf_item_copy_iovec(lv, vecp, bp, offset,
-                                               first_bit, nbits);
-                       blfp->blf_size++;
-                       break;
-               } else if (next_bit != last_bit + 1 ||
-                          xfs_buf_item_straddle(bp, offset, next_bit, 
last_bit)) {
-                       xfs_buf_item_copy_iovec(lv, vecp, bp, offset,
-                                               first_bit, nbits);
-                       blfp->blf_size++;
-                       first_bit = next_bit;
-                       last_bit = next_bit;
-                       nbits = 1;
-               } else {
-                       last_bit++;
-                       nbits++;
-               }
-       }
+       start = 0;
+       if (offset < rp->first)
+               start = rp->first - offset;
+       end = length - 1;
+       if (offset + length > rp->last)
+               end = rp->last - offset - 1;
+
+       start &= ~((1 << XFS_BLF_SHIFT) - 1);
+       first_bit = start >> XFS_BLF_SHIFT;
+       last_bit = end >> XFS_BLF_SHIFT;
+       nbits = last_bit - first_bit + 1;
+       bitmap_set((unsigned long *)blfp->blf_data_map, first_bit, nbits);
+
+       ASSERT(end <= length);
+       ASSERT(start <= length);
+       ASSERT(length >= nbits * XFS_BLF_CHUNK);
+       /*
+        * Copy needs to be done a buffer page at a time as we can be logging
+        * unmapped buffers. hence we have to use xfs_buf_iomove() rather than a
+        * straight memcpy here.
+        */
+       offset += first_bit * XFS_BLF_CHUNK;
+       length = nbits * XFS_BLF_CHUNK;
+       buf = xlog_prepare_iovec(lv, vecp, XLOG_REG_TYPE_BCHUNK);
+       xfs_buf_iomove(bp, offset, length, buf, XBRW_READ);
+       xlog_finish_iovec(lv, *vecp, length);
 }
 
 /*
@@ -323,7 +293,6 @@ xfs_buf_item_format(
               (xfs_blft_from_flags(&bip->__bli_format) > XFS_BLFT_UNKNOWN_BUF
                && xfs_blft_from_flags(&bip->__bli_format) < XFS_BLFT_MAX_BUF));
 
-
        /*
         * If it is an inode buffer, transfer the in-memory state to the
         * format flags and clear the in-memory state.
@@ -357,9 +326,17 @@ xfs_buf_item_format(
        }
 
        for (i = 0; i < bip->bli_format_count; i++) {
-               xfs_buf_item_format_segment(bip, lv, &vecp, offset,
-                                           &bip->bli_formats[i]);
-               offset += bp->b_maps[i].bm_len;
+               /*
+                * Only format dirty regions or stale buffers
+                */
+               struct xfs_bli_range *rp = &bip->bli_range[0];
+               if ((bip->bli_flags & XFS_BLI_STALE) ||
+                   (rp->first <= offset + BBTOB(bp->b_maps[i].bm_len) &&
+                    rp->last > offset))
+                       xfs_buf_item_format_segment(bip, lv, &vecp, rp, offset,
+                                                   BBTOB(bp->b_maps[i].bm_len),
+                                                   &bip->bli_formats[i]);
+               offset += BBTOB(bp->b_maps[i].bm_len);
        }
 
        /*
@@ -800,6 +777,9 @@ xfs_buf_item_init(
                bip->bli_formats[i].blf_map_size = map_size;
        }
 
+       BUILD_BUG_ON(XFS_BLI_RANGES != 1);
+       bip->bli_range[0].first = UINT_MAX;
+
        /*
         * Put the buf item into the list of items attached to the
         * buffer at the front.
@@ -809,91 +789,9 @@ xfs_buf_item_init(
        bp->b_fspriv = bip;
 }
 
-
 /*
  * Mark bytes first through last inclusive as dirty in the buf
- * item's bitmap.
- */
-static void
-xfs_buf_item_log_segment(
-       uint                    first,
-       uint                    last,
-       uint                    *map)
-{
-       uint            first_bit;
-       uint            last_bit;
-       uint            bits_to_set;
-       uint            bits_set;
-       uint            word_num;
-       uint            *wordp;
-       uint            bit;
-       uint            end_bit;
-       uint            mask;
-
-       /*
-        * Convert byte offsets to bit numbers.
-        */
-       first_bit = first >> XFS_BLF_SHIFT;
-       last_bit = last >> XFS_BLF_SHIFT;
-
-       /*
-        * Calculate the total number of bits to be set.
-        */
-       bits_to_set = last_bit - first_bit + 1;
-
-       /*
-        * Get a pointer to the first word in the bitmap
-        * to set a bit in.
-        */
-       word_num = first_bit >> BIT_TO_WORD_SHIFT;
-       wordp = &map[word_num];
-
-       /*
-        * Calculate the starting bit in the first word.
-        */
-       bit = first_bit & (uint)(NBWORD - 1);
-
-       /*
-        * First set any bits in the first word of our range.
-        * If it starts at bit 0 of the word, it will be
-        * set below rather than here.  That is what the variable
-        * bit tells us. The variable bits_set tracks the number
-        * of bits that have been set so far.  End_bit is the number
-        * of the last bit to be set in this word plus one.
-        */
-       if (bit) {
-               end_bit = MIN(bit + bits_to_set, (uint)NBWORD);
-               mask = ((1 << (end_bit - bit)) - 1) << bit;
-               *wordp |= mask;
-               wordp++;
-               bits_set = end_bit - bit;
-       } else {
-               bits_set = 0;
-       }
-
-       /*
-        * Now set bits a whole word at a time that are between
-        * first_bit and last_bit.
-        */
-       while ((bits_to_set - bits_set) >= NBWORD) {
-               *wordp |= 0xffffffff;
-               bits_set += NBWORD;
-               wordp++;
-       }
-
-       /*
-        * Finally, set any bits left to be set in one last partial word.
-        */
-       end_bit = bits_to_set - bits_set;
-       if (end_bit) {
-               mask = (1 << end_bit) - 1;
-               *wordp |= mask;
-       }
-}
-
-/*
- * Mark bytes first through last inclusive as dirty in the buf
- * item's bitmap.
+ * record dirty regions on the buffer.
  */
 void
 xfs_buf_item_log(
@@ -901,33 +799,19 @@ xfs_buf_item_log(
        uint                    first,
        uint                    last)
 {
-       int                     i;
-       uint                    start;
-       uint                    end;
-       struct xfs_buf          *bp = bip->bli_buf;
-
-       /*
-        * walk each buffer segment and mark them dirty appropriately.
-        */
-       start = 0;
-       for (i = 0; i < bip->bli_format_count; i++) {
-               if (start > last)
-                       break;
-               end = start + BBTOB(bp->b_maps[i].bm_len);
-               if (first > end) {
-                       start += BBTOB(bp->b_maps[i].bm_len);
-                       continue;
-               }
-               if (first < start)
-                       first = start;
-               if (end > last)
-                       end = last;
-
-               xfs_buf_item_log_segment(first, end,
-                                        &bip->bli_formats[i].blf_data_map[0]);
-
-               start += bp->b_maps[i].bm_len;
-       }
+       ASSERT(last != 0);
+       ASSERT(first <= last);
+       ASSERT(last < BBTOB(bip->bli_buf->b_length));
+
+       /* initial implementation - single range */
+       BUILD_BUG_ON(XFS_BLI_RANGES != 1);
+       if (first < bip->bli_range[0].first)
+               bip->bli_range[0].first = rounddown(first, XFS_BLF_CHUNK);
+       if (last > bip->bli_range[0].last)
+               bip->bli_range[0].last = roundup(last, XFS_BLF_CHUNK);
+
+       ASSERT(bip->bli_range[0].last != 0);
+       ASSERT(bip->bli_range[0].first <= bip->bli_range[0].last);
 }
 
 
diff --git a/fs/xfs/xfs_buf_item.h b/fs/xfs/xfs_buf_item.h
index 3f3455a..3594eb6 100644
--- a/fs/xfs/xfs_buf_item.h
+++ b/fs/xfs/xfs_buf_item.h
@@ -57,6 +57,25 @@ typedef struct xfs_buf_log_item {
        unsigned int            bli_recur;      /* lock recursion count */
        atomic_t                bli_refcount;   /* cnt of tp refs */
        int                     bli_format_count;       /* count of headers */
+
+       /*
+        * logging ranges. Keep a small number of distinct ranges rather than a
+        * bitmap which is expensive to maintain. Initial implementation is just
+        * a single range, but it is like that 3 or 4 will be optimal so that we
+        * can log separate header, tail and content changes (e.g. for dir
+        * structures) without capturing the entire buffer unnecessarily for
+        * isolated changes.
+        *
+        * Note: ranges are 32 bit values because we have to support an end
+        * range value of 0x10000....
+        */
+#define XFS_BLI_RANGES 1
+       struct xfs_bli_range {
+               uint32_t        first;
+               uint32_t        last;
+       }                       bli_range[XFS_BLI_RANGES];
+       int                     bli_ranges;
+
        struct xfs_buf_log_format *bli_formats; /* array of in-log header ptrs 
*/
        struct xfs_buf_log_format __bli_format; /* embedded in-log header */
 } xfs_buf_log_item_t;
-- 
2.0.0

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