[ltt-dev] [PATCH 01/10 round10] introduce bucket_at() and improve readability

[mathieu.desnoyers@efficios.com (Mathieu Desnoyers)]

* Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> On 11/22/2011 05:58 PM, Mathieu Desnoyers wrote:
> > * Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> >> Fast path is not changed.
> >> It will slow down very little for slow path.
> > 
> > it cleans up the code, so it's generally fine with me. There is just one
> > part that I don't understand, see below,
> > 
> >>
> >> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> >> ---
> >>  rculfhash.c |  100 +++++++++++++++++++++++++++-------------------------------
> >>  1 files changed, 47 insertions(+), 53 deletions(-)
> >>
> >> diff --git a/rculfhash.c b/rculfhash.c
> >> index bda3bd6..b0c7de5 100644
> >> --- a/rculfhash.c
> >> +++ b/rculfhash.c
> >> @@ -755,18 +755,14 @@ unsigned long _uatomic_xchg_monotonic_increase(unsigned long *ptr,
> >>  	return old2;
> >>  }
> >>  
> >> -static
> >> -struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
> >> -		unsigned long hash)
> >> +static inline
> >> +struct cds_lfht_node *bucket_at(struct cds_lfht *ht, unsigned long index)
> >>  {
> >> -	unsigned long index, order;
> >> -
> >> -	assert(size > 0);
> >> -	index = hash & (size - 1);
> >> +	unsigned long order;
> >>  
> >> -	if (index < ht->min_alloc_size) {
> >> -		dbg_printf("lookup hash %lu index %lu order 0 aridx 0\n",
> >> -			   hash, index);
> >> +	if ((__builtin_constant_p(index) && index == 0)
> >> +			|| index < ht->min_alloc_size) {
> >> +		dbg_printf("bucket index %lu order 0 aridx 0\n", index);
> >>  		return &ht->t.tbl[0]->nodes[index];
> >>  	}
> >>  	/*
> >> @@ -775,11 +771,19 @@ struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
> >>  	 * get_count_order_ulong.
> >>  	 */
> >>  	order = fls_ulong(index);
> >> -	dbg_printf("lookup hash %lu index %lu order %lu aridx %lu\n",
> >> -		   hash, index, order, index & ((1UL << (order - 1)) - 1));
> >> +	dbg_printf("bucket index %lu order %lu aridx %lu\n",
> >> +		   index, order, index & ((1UL << (order - 1)) - 1));
> >>  	return &ht->t.tbl[order]->nodes[index & ((1UL << (order - 1)) - 1)];
> >>  }
> >>  
> >> +static inline
> >> +struct cds_lfht_node *lookup_bucket(struct cds_lfht *ht, unsigned long size,
> >> +		unsigned long hash)
> >> +{
> >> +	assert(size > 0);
> >> +	return bucket_at(ht, hash & (size - 1));
> >> +}
> >> +
> >>  /*
> >>   * Remove all logically deleted nodes from a bucket up to a certain node key.
> >>   */
> >> @@ -1105,19 +1109,18 @@ static
> >>  void init_table_populate_partition(struct cds_lfht *ht, unsigned long i,
> >>  				   unsigned long start, unsigned long len)
> >>  {
> >> -	unsigned long j;
> >> +	unsigned long j, size = 1UL << (i - 1);
> >>  
> >>  	assert(i > ht->min_alloc_order);
> >>  	ht->cds_lfht_rcu_read_lock();
> >> -	for (j = start; j < start + len; j++) {
> >> -		struct cds_lfht_node *new_node = &ht->t.tbl[i]->nodes[j];
> >> -
> >> -		dbg_printf("init populate: i %lu j %lu hash %lu\n",
> >> -			   i, j, (1UL << (i - 1)) + j);
> >> -		new_node->reverse_hash =
> >> -				bit_reverse_ulong((1UL << (i - 1)) + j);
> >> -		_cds_lfht_add(ht, NULL, NULL, 1UL << (i - 1),
> >> -				new_node, NULL, 1);
> >> +	for (j = start + size; j < size + start + len; j++) {
> > 
> > Please use "j = size + start; j < size + start + len;"
> > 
> >> +		struct cds_lfht_node *new_node = bucket_at(ht, j);
> >> +
> >> +		assert(j >= size && j < (size << 1));
> >> +		dbg_printf("init populate: order %lu index %lu hash %lu\n",
> >> +			   i, j, j);
> >> +		new_node->reverse_hash = bit_reverse_ulong(j);
> >> +		_cds_lfht_add(ht, NULL, NULL, size, new_node, NULL, 1);
> >>  	}
> >>  	ht->cds_lfht_rcu_read_unlock();
> >>  }
> >> @@ -1205,18 +1208,18 @@ static
> >>  void remove_table_partition(struct cds_lfht *ht, unsigned long i,
> >>  			    unsigned long start, unsigned long len)
> >>  {
> >> -	unsigned long j;
> >> +	unsigned long j, size = 1UL << (i - 1);
> >>  
> >>  	assert(i > ht->min_alloc_order);
> >>  	ht->cds_lfht_rcu_read_lock();
> >> -	for (j = start; j < start + len; j++) {
> >> -		struct cds_lfht_node *fini_node = &ht->t.tbl[i]->nodes[j];
> >> -
> >> -		dbg_printf("remove entry: i %lu j %lu hash %lu\n",
> >> -			   i, j, (1UL << (i - 1)) + j);
> >> -		fini_node->reverse_hash =
> >> -			bit_reverse_ulong((1UL << (i - 1)) + j);
> >> -		(void) _cds_lfht_del(ht, 1UL << (i - 1), fini_node, 1);
> >> +	for (j = size + start; j < size + start + len; j++) {
> >> +		struct cds_lfht_node *fini_node = bucket_at(ht, j);
> >> +
> >> +		assert(j >= size && j < (size << 1));
> >> +		dbg_printf("remove entry: order %lu index %lu hash %lu\n",
> >> +			   i, j, j);
> >> +		fini_node->reverse_hash = bit_reverse_ulong(j);
> >> +		(void) _cds_lfht_del(ht, size, fini_node, 1);
> >>  	}
> >>  	ht->cds_lfht_rcu_read_unlock();
> >>  }
> >> @@ -1293,14 +1296,15 @@ static
> >>  void cds_lfht_create_bucket(struct cds_lfht *ht, unsigned long size)
> >>  {
> >>  	struct cds_lfht_node *prev, *node;
> >> -	unsigned long order, len, i, j;
> >> +	unsigned long order, len, i;
> >>  
> >>  	ht->t.tbl[0] = calloc(1, ht->min_alloc_size * sizeof(struct cds_lfht_node));
> >>  	assert(ht->t.tbl[0]);
> >>  
> >> -	dbg_printf("create bucket: order %lu index %lu hash %lu\n", 0, 0, 0);
> >> -	ht->t.tbl[0]->nodes[0].next = flag_bucket(get_end());
> >> -	ht->t.tbl[0]->nodes[0].reverse_hash = 0;
> >> +	dbg_printf("create bucket: order 0 index 0 hash 0\n");
> >> +	node = bucket_at(ht, 0);
> >> +	node->next = flag_bucket(get_end());
> >> +	node->reverse_hash = 0;
> >>  
> >>  	for (order = 1; order < get_count_order_ulong(size) + 1; order++) {
> >>  		len = 1UL << (order - 1);
> >> @@ -1311,22 +1315,15 @@ void cds_lfht_create_bucket(struct cds_lfht *ht, unsigned long size)
> >>  			assert(ht->t.tbl[order]);
> >>  		}
> >>  
> > 
> > below is the change I fail to understand. I think I'd need to be walked
> > through this change with an explanation, which could be added as a
> > comment telling what this function is doing.
> 
> The "if else if" code simulates the code of "set prev as the bucket with index=i".
> it is "bucket_at(ht, i)".
> 
> Now, we are trying to init the node with the hash=(i+len) (which is also
> a bucket with index=(i+len)) to the hash table, so this node has to be inserted
> after the bucket with index=i. And because there is no other non-bucket node
> nor bucket node with larger index inserted, so the bucket node with index=(i+len)
> should be inserted directly linked after the bucket node with index=i.
> 

Sounds like a very reasonable explanation to me. Please add this as a
comment block above the function.

Thanks!

Mathieu

> 
> > 
> >> -		i = 0;
> >> -		prev = ht->t.tbl[i]->nodes;
> >> -		for (j = 0; j < len; j++) {
> >> -			if (j & (j - 1)) {	/* Between power of 2 */
> >> -				prev++;
> >> -			} else if (j) {		/* At each power of 2 */
> >> -				i++;
> >> -				prev = ht->t.tbl[i]->nodes;
> >> -			}
> >> +		for (i = 0; i < len; i++) {
> >> +			prev = bucket_at(ht, i);
> >> +			node = bucket_at(ht, i + len);
> > 
> > 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com