On 2019/01/08 07:54:16 +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?
>
>> 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?
And the WRITE_ONCE() in the following line.
Thanks, Akira
>
> 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]
>>
>
