Re: [PATCH 25/26] block: Reduce zone write plugging memory usage

Bart Van Assche
2024-02-12T18:58:16Z

On 2/11/24 17:09, Damien Le Moal wrote:
> On 2/11/24 12:40, Bart Van Assche wrote:
>> For the use cases I'm interested in a hash table implementation that supports
>> RCU-lookups probably will work better than an xarray. I think that the hash
>> table implementation in <linux/hashtable.h> supports RCU for lookups, insertion
>> and removal.
>
> It does, but the API for it is not the easiest, and I do not see how that could
> be faster than an xarray, especially as the number of zones grows with high
> capacity devices (read: potentially more collisions which will slow zone plug
> lookups).

From the xarray documentation: "The XArray implementation is efficient when the
indices used are densely clustered". I think we are dealing with a sparse array
and hence that an xarray may not be the best suited data structure. How about
using a hash table and making the hash table larger if the number of open zones
equals the hash table size? That is possible as follows:
* Instead of using DEFINE_HASHTABLE() or DECLARE_HASHTABLE(), allocate the hash
table dynamically and use the struct hlist_head __rcu * data type.
* Use rcu_assign_pointer() to modify that pointer and kfree_rcu() to free old
versions of the hash table.
* Use rcu_dereference_protected() for hash table lookups.

For an example, see also the output of the following command:
$ git grep -nHw state_bydst

Thanks,

Bart.