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@xxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxx>
> >
> > 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]
> >
>
