Re: Question regarding hash_resize

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Tue, Jan 08, 2019 at 07:54:16AM +0900, Akira Yokosawa wrote:
> Hi Paul, 
> 
> On 2019/01/07 10:33:17 -0800, Paul E. McKenney wrote:
> > On Mon, Jan 07, 2019 at 09:49:19PM +0800, Junchang Wang wrote:
> >> Hi all,
> >>
> >> I'm reading hash_resize recently, and have a few questions regarding
> >> this algorithm. Please take a look if you have time. Any suggestions
> >> are warmly welcomed.
> >>
> >> === Question 1 ===
> >> In hash_resize.c : hashtab_lock_mod
> >> 186         if (b > READ_ONCE(htp->ht_resize_cur)) {
> >> 187                 lsp->hbp[1] = NULL;
> >> 188                 return;
> >> 189         }
> >> 190         htp = rcu_dereference(htp->ht_new);
> >>
> >> It seems we are missing a barrier (e.g., smp_mb) in between lines 189
> >> and 190, because neither READ_ONCE() nor rcu_dereference() can prevent
> >> compilers and hardware from reordering the two unrelated variables,
> >> ht_resize_cur and ht_new. Is my understanding correct?
> > 
> > Ah, but hashtab_lock_mod() is invoked within an RCU read-side critical
> > section
> 
> You mean "rcu_read_lock() at the beginning of hashtab_lock_mod() starts
> an RCU read-side critical section", don't you?

Indeed I do, good catch!

> >         and there is a synchronize_rcu() between the update to ->ht_new
> > and the updates to ->ht_resize_cur.  For more details on how this works,
> > please see https://lwn.net/Articles/573497/.
> > 
> > Of course, if you find a code path in which a call to hashtab_lock_mod()
> > is invoked outside of an RCU read-side critical section, that would be
> > a bug.  (Can you tell me an exception to this rule, that is, a case
> > where hashtab_lock_mod() could safely be invoked outside of an RCU
> > read-side critical section?)
> > 
> >> === Question 2 ===
> >> In hash_resize.c, each time an updater wants to access a bucket, the
> >> updater must first acquire the bucket's lock (htb_lock), preventing
> >> other updaters accessing the same bucket concurrently. This approach
> >> is OK if the linked list of a bucket is relatively short, but for a
> >> larger system where linked lists are long enough and the
> >> perftest_resize thread is running simultaneously, it could become a
> >> potential performance bottleneck. One naive solution is to allow
> >> multiple updaters to access the same bucket, only if they don't
> >> operate on the same item of the list of this bucket. I wonder if there
> >> are any existing works or discussions on this topic?
> > 
> > One approach is to use a hashed array of locks, and to hash a given
> > element's address to locate the lock to be used.  Please see
> > Section 7.1.1.5 ("Conditional Locking") and Section 7.1.1.6 ("Acquire
> > Needed Locks First"), including Quick Quiz 7.9, for additional details.
> > 
> > Another approach is to use RCU to protect traversals, and locks within the
> > linked-list elements themselves.  These locks are conditionally acquired
> > (again, please see Section 7.1.1.5), and deadlock is avoided by acquiring
> > them in list order, and the tricks in Quick Quiz 7.9.
> > 
> > Non-blocking synchronization can also be used, but it is often quite a
> > bit more complicated.  See for example the split-order list of Shalev
> > and Shavit, along with Desnoyers's RCU-protected extension in the
> > userspace RCU library.
> > 
> > But it is usually -way- better to just choose a good hash function and
> > to increase the number of buckets.  Which is of course one reason for
> > having resizable hash tables.  ;-)
> > 
> > But the other techniques can be useful in more complex linked data
> > structures, such as graphs, where there is no reasonable way to
> > partition the data.  Nevertheless, many people choose to do the
> > partitioning anyway, especially on distributed systems.
> > 
> >> === Question 3 ===
> >> Chapter Data Structures also discusses other resizable hash tables,
> >> namely "Resizable, scalable, concurrent hash tables via relativistic
> >> programming" from Josh Triplett, which can save memory footprint by
> >> using a single pair of pointers. But my understanding is that
> >> perftest_resize.c is unique in that it allows you to rebuild the hash
> >> table by utilizing a different hash function, which could be very
> >> useful in practice (e.g., to prevent DDoS attack). Other solutions do
> >> not share this property. Is my understanding correct? Did I miss any
> >> discussions on this topic in perfbook?
> > 
> > Indeed, to the best of my knowledge, Herbert Xu's pointer-pair approach
> > (which I use in hash_resize.c) is the only one allowing arbitrary changes
> > to hash functions.  I expect that this advantage will become increasingly
> > important as security issues become more challenging.  Furthermore, I
> > suspect that the pointer-pair approach is faster and more scalable.
> > It is certainly simpler.
> > 
> > On the other hand, one advantage of the other two approaches is decreased
> > memory consumption.
> > 
> > Another advantage of Josh Triplett's pointer-unzip approach is that
> > concurrent updates are (in theory, anyway) not blocked for as long
> > by resize operations.  The other edge of this sword is that resizing
> > is much slower, given the need to wait for many RCU grace periods.
> > 
> > Another advantage of Mathieu Desnoyers's RCUified variant of Shalev
> > and Shavit's split-order list is that all operations are non-blocking,
> > which can be important on massively overloaded systems, such as one
> > might find in cloud computing.
> > 
> >> === Question 4 ===
> >> In the current implementation of hash_resize.c, the perftest_resize
> >> could block an updater, and vice versa. It seems this is not what we
> >> expected. Ideally, they should be allowed to run concurrently, or at
> >> least the perftest_resize thread should have lower priority and
> >> updaters should never be blocked by the perftest_resize thread. Is
> >> that right? I'm very interested in helping improve. Please let me know
> >> if you have any suggestions.
> > 
> > In hash_resize.c, an updater is blocked only for the time required to
> > redisposition a bucket.  This is a great improvement over blocking
> > updaters for the full resize over all buckets.
> > 
> > But yes, it is not hard to do better, for example, periodically dropping
> > the old-table lock in hashtab_resize().  This requires a few careful
> > adjustments, of course.  Can you tell me what these adjustments are?
> > 
> > Hmmm...  I could simplify hashtab_lookup(), couldn't I?  After all,
> > optimizing for the race with hashtab_resize() doesn't make a whole lot
> > of sense.  Please see the patch below.  Thoughts?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 737646a9c868d841b32199b52f5569668975953e
> > Author: Paul E. McKenney <paulmck@linux.ibm.com>
> > Date:   Mon Jan 7 10:29:14 2019 -0800
> > 
> >     datastruct/hash: Simplify hashtab_lookup()
> >     
> >     Because resizing leaves the old hash table intact, and because lookups
> >     are carried out within RCU read-side critical sections (which prevent
> >     a second resizing operation from starting), there is no need for a
> >     lookup to search anywhere but in the old hash table.  And in the common
> >     case, there is no resize, so there is no new hash table.  Therefore,
> >     eliminating the check for resizing speeds things up in the common
> >     case.  In addition, this simplifies the code.
> >     
> >     This commit therefore eliminates the ht_get_bucket() function,
> >     renames the ht_get_bucket_single() function to ht_get_bucket(),
> >     and modifies callers appropriately.
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > 
> > diff --git a/CodeSamples/datastruct/hash/hash_resize.c b/CodeSamples/datastruct/hash/hash_resize.c
> > index 29e05f907200..be4157959b83 100644
> > --- a/CodeSamples/datastruct/hash/hash_resize.c
> > +++ b/CodeSamples/datastruct/hash/hash_resize.c
> > @@ -124,8 +124,7 @@ void hashtab_free(struct hashtab *htp_master)
> >  //\begin{snippet}[labelbase=ln:datastruct:hash_resize:get_bucket,commandchars=\\\@\$]
> >  /* Get hash bucket corresponding to key, ignoring the possibility of resize. */
> >  static struct ht_bucket *				//\lnlbl{single:b}
> > -ht_get_bucket_single(struct ht *htp, void *key, long *b,
> > -                     unsigned long *h)
> > +ht_get_bucket(struct ht *htp, void *key, long *b, unsigned long *h)
> >  {
> >  	unsigned long hash = htp->ht_gethash(key);
> >  
> > @@ -134,24 +133,6 @@ ht_get_bucket_single(struct ht *htp, void *key, long *b,
> >  		*h = hash;				//\lnlbl{single:h}
> >  	return &htp->ht_bkt[*b];			//\lnlbl{single:return}
> >  }							//\lnlbl{single:e}
> > -
> > -/* Get hash bucket correesponding to key, accounting for resize. */
> > -static struct ht_bucket *				//\lnlbl{b}
> > -ht_get_bucket(struct ht **htp, void *key, long *b, int *i)
> > -{
> > -	struct ht_bucket *htbp;
> > -
> > -	htbp = ht_get_bucket_single(*htp, key, b, NULL); //\lnlbl{call_single}
> > -								//\fcvexclude
> > -	if (*b <= READ_ONCE((*htp)->ht_resize_cur)) {	//\lnlbl{resized}
> > -		smp_mb(); /* order ->ht_resize_cur before ->ht_new. */
> 
> If we can remove this memory barrier, the counterpart smp_mb() in
> hashtab_resize() becomes unnecessary, doesn't it?

Or maybe I need to add a memory barrier to hashtab_lock_mod().  Thoughts?

							Thanx, Paul

>         Thanks, Akira
> 
> > -		*htp = rcu_dereference((*htp)->ht_new);	//\lnlbl{newtable}
> > -		htbp = ht_get_bucket_single(*htp, key, b, NULL); //\lnlbl{newbucket}
> > -	}
> > -	if (i)						//\lnlbl{chk_i}
> > -		*i = (*htp)->ht_idx;			//\lnlbl{set_idx}
> > -	return htbp;					//\lnlbl{return}
> > -}							//\lnlbl{e}
> >  //\end{snippet}
> >  
> >  /* Read-side lock/unlock functions. */
> > @@ -178,7 +159,7 @@ hashtab_lock_mod(struct hashtab *htp_master, void *key,
> >  
> >  	rcu_read_lock();				//\lnlbl{l:rcu_lock}
> >  	htp = rcu_dereference(htp_master->ht_cur);	//\lnlbl{l:refhashtbl}
> > -	htbp = ht_get_bucket_single(htp, key, &b, &h);	//\lnlbl{l:refbucket}
> > +	htbp = ht_get_bucket(htp, key, &b, &h);		//\lnlbl{l:refbucket}
> >  	spin_lock(&htbp->htb_lock);			//\lnlbl{l:acq_bucket}
> >  	lsp->hbp[0] = htbp;				//\lnlbl{l:lsp0b}
> >  	lsp->hls_idx[0] = htp->ht_idx;
> > @@ -188,7 +169,7 @@ hashtab_lock_mod(struct hashtab *htp_master, void *key,
> >  		return;					//\lnlbl{l:fastret1}
> >  	}
> >  	htp = rcu_dereference(htp->ht_new);		//\lnlbl{l:new_hashtbl}
> > -	htbp = ht_get_bucket_single(htp, key, &b, &h);	//\lnlbl{l:get_newbkt}
> > +	htbp = ht_get_bucket(htp, key, &b, &h);		//\lnlbl{l:get_newbkt}
> >  	spin_lock(&htbp->htb_lock);			//\lnlbl{l:acq_newbkt}
> >  	lsp->hbp[1] = htbp;				//\lnlbl{l:lsp1b}
> >  	lsp->hls_idx[1] = htp->ht_idx;
> > @@ -223,16 +204,15 @@ struct ht_elem *					//\lnlbl{lkp:b}
> >  hashtab_lookup(struct hashtab *htp_master, void *key)
> >  {
> >  	long b;
> > -	int i;
> >  	struct ht *htp;
> >  	struct ht_elem *htep;
> >  	struct ht_bucket *htbp;
> >  
> >  	htp = rcu_dereference(htp_master->ht_cur);	//\lnlbl{lkp:get_curtbl}
> > -	htbp = ht_get_bucket(&htp, key, &b, &i);	//\lnlbl{lkp:get_curbkt}
> > +	htbp = ht_get_bucket(htp, key, &b, NULL);	//\lnlbl{lkp:get_curbkt}
> >  	cds_list_for_each_entry_rcu(htep,		//\lnlbl{lkp:loop:b}
> >  	                            &htbp->htb_head,
> > -	                            hte_next[i]) {
> > +	                            hte_next[htp->ht_idx]) {
> >  		if (htp->ht_cmp(htep, key)) 		//\lnlbl{lkp:match}
> >  			return htep;			//\lnlbl{lkp:ret_match}
> >  	}						//\lnlbl{lkp:loop:e}
> > @@ -303,7 +283,7 @@ int hashtab_resize(struct hashtab *htp_master,
> >  		htbp = &htp->ht_bkt[i];			//\lnlbl{get_oldcur}
> >  		spin_lock(&htbp->htb_lock);		//\lnlbl{acq_oldcur}
> >  		cds_list_for_each_entry(htep, &htbp->htb_head, hte_next[idx]) { //\lnlbl{loop_list:b}
> > -			htbp_new = ht_get_bucket_single(htp_new, htp_new->ht_getkey(htep), &b, NULL);
> > +			htbp_new = ht_get_bucket(htp_new, htp_new->ht_getkey(htep), &b, NULL);
> >  			spin_lock(&htbp_new->htb_lock);
> >  			cds_list_add_rcu(&htep->hte_next[!idx], &htbp_new->htb_head);
> >  			spin_unlock(&htbp_new->htb_lock);
> > diff --git a/datastruct/datastruct.tex b/datastruct/datastruct.tex
> > index 5c61bf5e2389..0152437c274e 100644
> > --- a/datastruct/datastruct.tex
> > +++ b/datastruct/datastruct.tex
> > @@ -966,10 +966,8 @@ the old table.
> >  \begin{lineref}[ln:datastruct:hash_resize:get_bucket]
> >  Bucket selection is shown in
> >  Listing~\ref{lst:datastruct:Resizable Hash-Table Bucket Selection},
> > -which shows \co{ht_get_bucket_single()} on
> > -lines~\lnref{single:b}-\lnref{single:e} and
> > -\co{ht_get_bucket()} on lines~\lnref{b}-\lnref{e}.
> > -The \co{ht_get_bucket_single()} function returns a reference to the bucket
> > +which shows \co{ht_get_bucket()}.
> > +This function returns a reference to the bucket
> >  corresponding to the specified key in the specified hash table, without
> >  making any allowances for resizing.
> >  It also stores the bucket index corresponding to the key into the location
> > @@ -978,36 +976,6 @@ line~\lnref{single:gethash}, and the corresponding
> >  hash value corresponding to the key into the location
> >  referenced by parameter~\co{h} (if non-\co{NULL}) on line~\lnref{single:h}.
> >  Line~\lnref{single:return} then returns a reference to the corresponding bucket.
> > -
> > -The \co{ht_get_bucket()} function handles hash-table selection, invoking
> > -\co{ht_get_bucket_single()} on
> > -line~\lnref{call_single} to select the bucket
> > -corresponding to the hash in the current
> > -hash table, storing the hash value through parameter~\co{b}.
> > -If line~\lnref{resized} determines that the table is being resized and that
> > -line~\lnref{call_single}'s bucket has already been distributed across the new hash
> > -table, then line~\lnref{newtable} selects the new hash table and
> > -line~\lnref{newbucket}
> > -selects the bucket corresponding to the hash in the new hash table,
> > -again storing the hash value through parameter~\co{b}.
> > -\end{lineref}
> > -
> > -\QuickQuiz{}
> > -	The code in
> > -	Listing~\ref{lst:datastruct:Resizable Hash-Table Bucket Selection}
> > -	computes the hash twice!
> > -	Why this blatant inefficiency?
> > -\QuickQuizAnswer{
> > -	The reason is that the old and new hash tables might have
> > -	completely different hash functions, so that a hash computed
> > -	for the old table might be completely irrelevant to the
> > -	new table.
> > -} \QuickQuizEnd
> > -
> > -\begin{lineref}[ln:datastruct:hash_resize:get_bucket]
> > -If line~\lnref{chk_i} finds that parameter~\co{i} is non-\co{NULL}, then
> > -line~\lnref{set_idx} stores the pointer-set index for the selected hash table.
> > -Finally, line~\lnref{return} returns a reference to the selected hash bucket.
> >  \end{lineref}
> >  
> >  \QuickQuiz{}
> > @@ -1021,10 +989,8 @@ Finally, line~\lnref{return} returns a reference to the selected hash bucket.
> >  	functions described next.
> >  } \QuickQuizEnd
> >  
> > -This implementation of
> > -\co{ht_get_bucket_single()} and \co{ht_get_bucket()}
> > -permit lookups and modifications to run concurrently
> > -with a resize operation.
> > +This implementation of \co{ht_get_bucket()} permits lookups and
> > +modifications to run concurrently with a resize operation.
> >  
> >  \begin{listing}[tb]
> >  \input{CodeSamples/datastruct/hash/hash_resize@lock_unlock_mod.fcv}
> > @@ -1129,11 +1095,6 @@ hash lookups.
> >  Line~\lnref{get_curtbl} fetches the current hash table and
> >  line~\lnref{get_curbkt} obtains a reference
> >  to the bucket corresponding to the specified key.
> > -This bucket will be located in a new resized hash table when a
> > -resize operation has progressed past the bucket in the old hash
> > -table that contained the desired data element.
> > -Note that line~\lnref{get_curbkt} also passes back the index that will be
> > -used to select the correct set of pointers from the pair in each element.
> >  The loop spanning lines~\lnref{loop:b}-\lnref{loop:e} searches the bucket,
> >  so that if line~\lnref{match}
> >  detects a match,
> > @@ -1144,22 +1105,17 @@ failure.
> >  \end{lineref}
> >  
> >  \QuickQuiz{}
> > -	In the \co{hashtab_lookup()} function in
> > -	Listing~\ref{lst:datastruct:Resizable Hash-Table Access Functions},
> > -	the code carefully finds the right bucket in the new hash table
> > -	if the element to be looked up has already been distributed
> > -	by a concurrent resize operation.
> > -	This seems wasteful for RCU-protected lookups.
> > -	Why not just stick with the old hash table in this case?
> > +	\begin{lineref}[ln:datastruct:hash_resize:access:lkp]
> > +	What if execution reaches line~\lnref{loop:b}
> > +	of \co{hashtab_lookup()} in
> > +	Listing~\ref{lst:datastruct:Resizable Hash-Table Access Functions}
> > +	just after this bucket has been resized.
> > +	Won't that result in lookup failures?
> > +	\end{lineref}
> >  \QuickQuizAnswer{
> > -	Suppose that a resize operation begins and distributes half of
> > -	the old table's buckets to the new table.
> > -	Suppose further that a thread adds a new element that goes into
> > -	one of the already-distributed buckets, and that this same thread
> > -	now looks up this newly added element.
> > -	If lookups unconditionally traversed only the old hash table,
> > -	this thread would get a lookup failure for the element that it
> > -	just added, which certainly sounds like a bug to me!
> > +	No, it won't.
> > +	Resizing into the new hash table leaves the old hash table
> > +	intact, courtesy of the pointer pairs.
> >  } \QuickQuizEnd
> >  
> >  \begin{lineref}[ln:datastruct:hash_resize:access:add]
> > 
>