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?
> 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?
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]
>
