On Fri, May 12, 2023 at 06:57:52PM -0700, Eric Biggers wrote:
> First, I wanted to mention that decoding of variable-length fields has been
> extensively studied for decompression algorithms, e.g. for Huffman decoding.
> And it turns out that it can be done branchlessly. The basic idea is that you
> have a branchless refill step that looks like the following:
>
> #define REFILL_BITS_BRANCHLESS() \
> bitbuf |= get_unaligned_u64(p) << bitsleft; \
> p += 7 - ((bitsleft >> 3) & 0x7); \
> bitsleft |= 56;
>
> That branchlessly ensures that 'bitbuf' contains '56 <= bitsleft <= 63' bits.
> Then, the needed number of bits can be removed and returned:
>
> #define READ_BITS(n) \
> REFILL_BITS_BRANCHLESS(); \
> tmp = bitbuf & (((u64)1 << (n)) - 1); \
> bitbuf >>= (n); \
> bitsleft -= (n); \
> tmp
>
> If you're interested, I can give you some references about the above method.
I might be interested in those references, new bit tricks and integer
encodings are always fun :)
> But, I really just wanted to mention it for completeness, since I think you'd
> actually want to go in a slightly different direction, since (a) you have all
> the field widths available from the beginning, as opposed to being interleaved
> into the bitstream itself (as is the case in Huffman decoding for example), so
> you're not limited to serialized decoding of each field, (b) your fields are up
> to 96 bits, and (c) you've selected a bitstream convention that seems to make it
> such that your stream *must* be read in aligned units of u64, so I don't think
> something like REFILL_BITS_BRANCHLESS() could work for you anyway.
>
> What I would suggest instead is preprocessing the list of 6 field lengths to
> create some information that can be used to extract all 6 fields branchlessly
> with no dependencies between different fields. (And you clearly *can* add a
> preprocessing step, as you already have one -- the dynamic code generator.)
>
> So, something like the following:
>
> const struct field_info *info = &format->fields[0];
>
> field0 = (in->u64s[info->word_idx] >> info->shift1) & info->mask;
> field0 |= in->u64s[info->word_idx - 1] >> info->shift2;
>
> ... but with the code for all 6 fields interleaved.
>
> On modern CPUs, I think that would be faster than your current C code.
>
> You could do better by creating variants that are specialized for specific
> common sets of parameters. During "preprocessing", you would select a variant
> and set an enum accordingly. During decoding, you would switch on that enum and
> call the appropriate variant. (This could also be done with a function pointer,
> of course, but indirect calls are slow these days...)
testing random btree updates:
dynamically generated unpack:
rand_insert: 20.0 MiB with 1 threads in 33 sec, 1609 nsec per iter, 607 KiB per sec
old C unpack:
rand_insert: 20.0 MiB with 1 threads in 35 sec, 1672 nsec per iter, 584 KiB per sec
the Eric Biggers special:
rand_insert: 20.0 MiB with 1 threads in 35 sec, 1676 nsec per iter, 583 KiB per sec
Tested two versions of your approach, one without a shift value, one
where we use a shift value to try to avoid unaligned access - second was
perhaps 1% faster
so it's not looking good. This benchmark doesn't even hit on
unpack_key() quite as much as I thought, so the difference is
significant.
diff --git a/fs/bcachefs/bkey.c b/fs/bcachefs/bkey.c
index 6d3a1c096f..128d96766c 100644
--- a/fs/bcachefs/bkey.c
+++ b/fs/bcachefs/bkey.c
@@ -7,6 +7,8 @@
#include "bset.h"
#include "util.h"
+#include <asm/unaligned.h>
+
#undef EBUG_ON
#ifdef DEBUG_BKEYS
@@ -19,9 +21,35 @@ const struct bkey_format bch2_bkey_format_current = BKEY_FORMAT_CURRENT;
struct bkey_format_processed bch2_bkey_format_postprocess(const struct bkey_format f)
{
- return (struct bkey_format_processed) {
- .f = f,
- };
+ struct bkey_format_processed ret = { .f = f, .aligned = true };
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+ unsigned offset = f.key_u64s * 64;
+#else
+ unsigned offset = KEY_PACKED_BITS_START;
+#endif
+
+ for (unsigned i = 0; i < BKEY_NR_FIELDS; i++) {
+ unsigned bits = f.bits_per_field[i];
+
+ if (bits & 7) {
+ ret.aligned = false;
+ break;
+ }
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+ offset -= bits;
+#endif
+
+ ret.shift[i] = min(offset & 63, 64 - bits);
+ ret.offset[i] = (offset - ret.shift[i]) / 8;
+ ret.mask[i] = bits ? ~0ULL >> (64 - bits) : 0;
+
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+ offset += bits;
+#endif
+ }
+
+ return ret;
}
void bch2_bkey_packed_to_binary_text(struct printbuf *out,
@@ -191,6 +219,19 @@ static u64 get_inc_field(struct unpack_state *state, unsigned field)
return v + offset;
}
+__always_inline
+static u64 get_aligned_field(const struct bkey_format_processed *f,
+ const struct bkey_packed *in,
+ unsigned field_idx)
+{
+ u64 v = get_unaligned((u64 *) (((u8 *) in->_data) + f->offset[field_idx]));
+
+ v >>= f->shift[field_idx];
+ v &= f->mask[field_idx];
+
+ return v + le64_to_cpu(f->f.field_offset[field_idx]);
+}
+
__always_inline
static bool set_inc_field(struct pack_state *state, unsigned field, u64 v)
{
@@ -269,45 +310,57 @@ bool bch2_bkey_transform(const struct bkey_format *out_f,
return true;
}
-struct bkey __bch2_bkey_unpack_key(const struct bkey_format_processed *format_p,
+struct bkey __bch2_bkey_unpack_key(const struct bkey_format_processed *format,
const struct bkey_packed *in)
{
- const struct bkey_format *format = &format_p->f;
- struct unpack_state state = unpack_state_init(format, in);
struct bkey out;
- EBUG_ON(format->nr_fields != BKEY_NR_FIELDS);
- EBUG_ON(in->u64s < format->key_u64s);
+ EBUG_ON(format->f.nr_fields != BKEY_NR_FIELDS);
+ EBUG_ON(in->u64s < format->f.key_u64s);
EBUG_ON(in->format != KEY_FORMAT_LOCAL_BTREE);
- EBUG_ON(in->u64s - format->key_u64s + BKEY_U64s > U8_MAX);
+ EBUG_ON(in->u64s - format->f.key_u64s + BKEY_U64s > U8_MAX);
- out.u64s = BKEY_U64s + in->u64s - format->key_u64s;
+ out.u64s = BKEY_U64s + in->u64s - format->f.key_u64s;
out.format = KEY_FORMAT_CURRENT;
out.needs_whiteout = in->needs_whiteout;
out.type = in->type;
out.pad[0] = 0;
+ if (likely(format->aligned)) {
+#define x(id, field) out.field = get_aligned_field(format, in, id);
+ bkey_fields()
+#undef x
+ } else {
+ struct unpack_state state = unpack_state_init(&format->f, in);
+
#define x(id, field) out.field = get_inc_field(&state, id);
- bkey_fields()
+ bkey_fields()
#undef x
+ }
return out;
}
-struct bpos __bkey_unpack_pos(const struct bkey_format_processed *format_p,
+struct bpos __bkey_unpack_pos(const struct bkey_format_processed *format,
const struct bkey_packed *in)
{
- const struct bkey_format *format = &format_p->f;
- struct unpack_state state = unpack_state_init(format, in);
struct bpos out;
- EBUG_ON(format->nr_fields != BKEY_NR_FIELDS);
- EBUG_ON(in->u64s < format->key_u64s);
+ EBUG_ON(format->f.nr_fields != BKEY_NR_FIELDS);
+ EBUG_ON(in->u64s < format->f.key_u64s);
EBUG_ON(in->format != KEY_FORMAT_LOCAL_BTREE);
- out.inode = get_inc_field(&state, BKEY_FIELD_INODE);
- out.offset = get_inc_field(&state, BKEY_FIELD_OFFSET);
- out.snapshot = get_inc_field(&state, BKEY_FIELD_SNAPSHOT);
+ if (likely(format->aligned)) {
+ out.inode = get_aligned_field(format, in, BKEY_FIELD_INODE);
+ out.offset = get_aligned_field(format, in, BKEY_FIELD_OFFSET);
+ out.snapshot = get_aligned_field(format, in, BKEY_FIELD_SNAPSHOT);
+ } else {
+ struct unpack_state state = unpack_state_init(&format->f, in);
+
+ out.inode = get_inc_field(&state, BKEY_FIELD_INODE);
+ out.offset = get_inc_field(&state, BKEY_FIELD_OFFSET);
+ out.snapshot = get_inc_field(&state, BKEY_FIELD_SNAPSHOT);
+ }
return out;
}
diff --git a/fs/bcachefs/btree_types.h b/fs/bcachefs/btree_types.h
index 58ce60c37e..38c3ec6852 100644
--- a/fs/bcachefs/btree_types.h
+++ b/fs/bcachefs/btree_types.h
@@ -70,6 +70,10 @@ struct btree_bkey_cached_common {
struct bkey_format_processed {
struct bkey_format f;
+ bool aligned;
+ u8 offset[6];
+ u8 shift[6];
+ u64 mask[6];
};
struct btree {
diff --git a/fs/bcachefs/btree_update_interior.h b/fs/bcachefs/btree_update_interior.h
index dcfd7ceacc..72aedc1e34 100644
--- a/fs/bcachefs/btree_update_interior.h
+++ b/fs/bcachefs/btree_update_interior.h
@@ -181,7 +181,11 @@ static inline void btree_node_reset_sib_u64s(struct btree *b)
static inline void *btree_data_end(struct bch_fs *c, struct btree *b)
{
- return (void *) b->data + btree_bytes(c);
+ /*
+ * __bch2_bkey_unpack_key() may read up to 8 bytes past the end of the
+ * input buffer:
+ */
+ return (void *) b->data + btree_bytes(c) - 8;
}
static inline struct bkey_packed *unwritten_whiteouts_start(struct bch_fs *c,
