Hash Container Notes

Currently there is a plethora of hash table in the Traffic Server core. One the initiatives from the Cork was to attempt to reduce this number. Before doing that successfully it is necessary to understand the uses cases inside Traffic Server.

Use Cases

Simple Hash Table
Use of a hash table for rapid lookup of stored items.
Concurrent Hash Table
The simple hash table case with the additional requirement concurrency. In general this means at most one writer and an arbitrary number of reads without explicit locks. Multiple writers are handled by an explicit lock which is required only for table updates, not for searching or reading.
Multi-index hash tables
In a few cases the items stored in the table need to be accessed via multiple distinct keys. The archetypical example is the upstream session table which needs to be keyed by FQDN and IP address.
External memory management
Because of the heavy use of proxy and class allocators, it is sometimes the case that memory and life time management of objects is independent of the hash container. Again, the upstream session table is archetypical in that sessions are created and destroyed elsewhere, outside the scope of the container). Therefore inserting and removing an object should not cause its creation or destruction. While it would be possible to use a container of smart pointers, this means allocating and deallocating nodes in the container for inserts and deletes, in contrast to an intrusive container which does not require such memory management.

A side property which is not required but is very conventient is the ability to use a member of the object as a key. While the key can be split off, this tends to tie the object to the container. Alternatively the key can be duplicated but if key requires local storage this requires another memory allocation for each object created.

Designs

There are two basic approaches to supporting hash containers in Traffic Server.

Standard Template Library

The Standard Template Library provides the containers std::unordered_set and std::unordered_map. These suffice for basic hash container use. The main concerns with the STL containers are

  1. Object lifetimes and memory management are tied to the container. This does not interact well with proxy allocators for reasons noted above.
  2. Relatedly, memory management is “invisible” and therfore tempting to use in places where allocations should be avoided. It is easy to use STL containers in ways that have very bad memory use characteristics.

The primary concern here, as I understand it, is that use of STL containers for specific purposes will encourage the same use in general, leading to significant performance degradation. This is not an idle concern - I am currently involved inside Oath with another project, a plugin for Traffic Server, which manages to consume as much CPU as the rest of Traffic Server put together due to uninformed choices in using STL containers.

As noted previously, the memory management issues could be worked around using smart pointers. However this would create a significant amount of additional memory management activity as the nodes in the container will need to be allocated and released with every insert and removal.

Based on research in to allocators for STL containers, it does look like this is possible. A major change in this area for C++11 is allocators can now be stateful. This is critical to prevent memory migration between two containers, e.g. if memory is allocated from an allocator for container A, it is not used for container B. However, currently this requires an update to MemArena which is blocked.

Intrusive Containers

Traffic Server has to a large extent rolled its own containers, primarily intrusive ones where the objects directly support the containers. This is likely due to the style of memory management where objects are allocatedand released independently of containers. This was originally a set of what became legacy containers in the header file “lib/ts/Map.h”. These lacked not only documentation but any compliance with STL standards, such as iterators, and handled only pointers. The containers also depended on the Vec class which is being phased out. For this reason, and to support multi-indexing, the class TSHashTable was created. TSHahhTable had the advantages of

  • Contain an arbitrary type T as long as it had intrusive list support.
  • Control of the key.
  • Control of bucket expansion policy.
  • Iteration.

These features make the container much easier for a standard C++ programmer to use as it enables following standard STL use patterns.

The ability to select the key is valuable for performance as well because IntrusiveHashMap takes care to never default construct a key, but only initialize them. In addition the comparison operator is under the control of the template parameters. This means the type used by the container for search may be a reference or pointer to the actual key. This allows the type in the object to own its memory while not imposing that requirement on the search key. For example, if the key is a std::string then the key used for searching can be const std::string & or even a std::string_view, with the result that searching the container does not require instantiating a std::string and the attendant memory management. For an STL container, searching requires creating a instance of key type and therefore also any memory allocation that construction requires. There have been attempts to work around this by using STL containers and making the key a reference or even a std::string_view in to the object but these have failed with obscure memory corruption issues. At present this is not a viable option. Additionally, it can appear that lookups can be done on an STL container using such a reference, but type coercion will cause the creation of the key transparently. This is a big deal for Traffic Server because in many cases the source key is a string that is embedded in other memory (e.g., the name of a field in an HTTP header).

Selecting the key also enabled the multi-index capability in that each index can select a different key. In addition, if using STL containers then the general solution is to have one owner container which has the actual objects and ancillary ones that contain pointers or iterators to the object in the owner container. The problem here is that one must either duplicate the keys (for the ancillary indices and in the object) or not have a key value available from other indices. This issue doesn’t arise in a container that allows the key to be embedded in the object.

With the switch to C++11 and then C++17, it seemed reasonable to update this class to take advantage of the new language features and become more STL compliant. This resulted in IntrusiveHashMap which has the advantages

  • Use of IntrusiveDList, a pure C++ class, instead of the C macro based linked lists used previously.
  • STL compliant support for ranges.
  • Full iterator support, replacing the psuedo-iterator TSHashTable::Location.
  • Simplified specification, reducing the number of elements required in the descriptor type.

IntrusiveHashMap has been through several iterations and example uses have be provided in pull requests. The intention is for IntrusiveHashMap to completely replace TSHashTable and uses of the other hash maps in “lib/ts/Map.h”. If the necessary support in MemArena (currently blocked) can be made available then IntrusiveHashMap could be used to replace the TCL hash tables.

IntrusiveHashMap works well with the standard allocation style used in Traffic Server. This will make it relatively easy to replace current uses of internal legacy hash containers with

IntrusiveHashMap satisfies the specialized requirements for multi-indexed containers. Given that, in my view, we will want this class for this reason, it seems a low marginal cost effort to use the class in the other situations mentioned above. Not doing so will not remove the need for this class.

Concurrency

Concurrency is hard to get right and fast. Currently there are custom concurrent hash containers in Traffic Server. These use the technique of splitting locks per bucket, or using a pool locks [1]. The alternative recommended at Cork is to use a well seasoned third party concurrent hash container. The three options are

  1. Thread Building Blocks
  2. Concurrent Data Structures
  3. Concurrency Kit

Each of these has issues from the Traffic Server point of view but, in my opinion, any would be a improvement over the current state of concurrent containers.

Thread Building Blocks
This is sponsored by Intel, but it is a heavy weight solution designed for a different domain (compute intensive data processing) than that of Traffic Server (event driven I/O processing). Use of TBB depends on being able to use just the concurrent containes and not most of the library. TBB is also under development and would require periodic updates.
Concurrent Data Structures
This is a nice set of concurrent containers although the documentation is a bit thin. The main problem here, in my view, is its dependency on thread support. It is unclear to me why this is necessary, but the main thread and each thread that uses the library must explicitly “attach” to the library in the thread. The development status is unknown.
Concurrency Kit
This is a very mature, well tested and fast set of concurrency containers. The problem here is it is written in C and not amenable to easy conversion to C++11. In fact, the C code is such that it will not even compile as C++ without extension re-skinning. It has been discussed to start a “CK++” project to do this, which was endorsed by the Concurrency Kit author. The base C version is a completed project, there is no active development.

Conclusions

While the various merits and costs of the various concurrent container libraries can be debated, the reality is likely we will adopt which ever of these is first brought in to compatibility with Traffic Server. My personal preference is for CK++, the reskinning of Concurrency Kit. However, I expect any of the choices will be good enough.

For uses cases that do not require concurrency I think concurrent containers should not be used. There is a cost to using them and why pay that if it’s not necessary? Concurrent containers should be used iff there is a clear reduction in use of an explicit lock. E.g. using a concurrent hash container means that read operations no longer need to get a lock.

For general hash containers there are two issues, ease of use and performance.

I think for our use case, where many objects are allocated via proxy allocators, the intrusive containers are a clear win. They clearly do less work, because they do not do any memory management. On the other hand, there are many uses of hash containers where either performance isn’t critical (e.g.. process static tables) or where the objects aren’t proxy allocated. In such cases I see no reason to not use STL containers, other than the carry on effects noted above. There is also the argument that use of jemalloc or even modern glibc provides the equivalent of per thread proxy allocators without the explicit coding requirements of the Traffic Server internal ones and therefore the performance differences will be minimal. I don’t consider that argument because the concensus, as I understand it, is that for the forseeable future the code base will keep the proxy allocators.

I find the ease of use argument comes to the same point. For proxy allocated objects, for multi-indexing situations, and where the key is naturally embedded in the object, IntrusiveHashMap is easier to use. I have for decades thought the require separation of key and object in STL containers is a problem to be worked around, not a feature, and that’s no small part of why I like IntrusiveHashMap better. No small part of the recent upgrade was to make the class more STL compliant so that it acts like an STL container, e.g. range for loops work as with an STL container. I agree that for cases where the object exists only in the container, the STL containers are the easier to use choice. It is also the case that IntrusiveHashmap requires the key to be in the object. This is usually reasonable but can be a problem in some cases. Obviously those are ones where an STL container is a better choice.

Overall, then, for basic hash containers I would have

  • IntrusiveHashMap for high performance use cases, and ease of use for the noted use cases.
  • Use STL containers for process statics or non-performant code. I think it should be OK to make this the default choice and use either IntrusiveHashMap or a concurrent container if a specific need to do so exists.

Footnotes

[1]In some sense these aren’t different - if the mutex in the pool is selected by key hashing it is effectively the same as a mutex per bucket.