Sunday, October 6, 2019

Recovery on a hand-made durable data structure versus recovery on a Persistent Transactional Memory

A durable data structure is a data structure in Persistent Memory (PM) or Non-Volatile Memory, if you prefer.
These data structures have to be resilient to failures. In other words, if there is a power cut during an operation on the data structure, the data structure must be correctly recovered after the system restarts.
For the rest of this post, and likely for all other posts you read in this site, when I say "durable data structure" I mean a data structure that has "atomic durability": In the event of a failure, the side effects of all completed operations will be visible upon restart and recovery.

The are two main approaches to having a durable data structure in PM:
1) Write an algorithm for a new data structure. This algorithm has to be failure-resilient;
2) Write a regular data structure and wrap it in a PTM;

Making a new data structure is typically the subject of an entire research paper, while using a PTM is not. Researchers tend to lean on doing new data structures because it's more fun (we can't get a paper accepted where we describe "using" a data structure). Which approach is better depends on what you mean by "better", but my favorite is 2).

However, set's say you decide to use a hand-made durable data structure that someone else made. Now how do you use it in your program?

Most applications have many data structure instances. For an application that needs durability (or persistence, if you prefer) not all data needs to be persistent, which means that not all data structures need to be persistent either. However, let's assume it is more than 1 data structure instance.
When a crash occurs, it could have occurred during an operation on *any* of these data structures. This means we need to call the recovery() method for each of these data structure instances, just in case.

This by itself creates a nuisance, because now you need to have an explicit list of all the durable data structures in your application and iterate through all in your recovery code:
void recovery() {   // called after a restart

If you forget to add one of the data structures to the main recovery method, you risk doing operations on an inconsistent data structure and further corrupting your data. In PM, when you corrupt the data it's gone forever... unless you had backups, and even then you're going to lose all data from the last non-corrupt backup.

Another difficulty is that sometimes the number of durable data structures may be dynamic because the application creates and destroys data structures during execution, to store some data. This means that it's not possible to add each data structure instance in the recovery() method because they may not exist when the program restarts. This typically means you will need a registration mechanism where newly created data structure instances are added and later de-registered when the data structure is destroyed. Also, you need to save the root pointer to each of those data structure instances.
PTMs handle this transparently because the transaction has the all-or-nothing semantics. If new data structure instances are created or destroyed, you can use the root pointer API (in OneFile and Romulus we called it get_object() and put_object()). When there is a crash, any modifications to ongoing transactions will be reverted, including allocating or de-allocating a new data structure instance and including adding or removing a root pointer. By having a PTM whose transactions can encompass the modifications on the data structure and the allocation/de-allocation of the data structure and the adding/removing of the instance as a root pointer, we give transactions that are easy to reason about and make your code consistent at all times.

Ok, ok, so it's not that bad for the hand-made durable data structures. We can do automatic checks for these scenarios by adding registration mechanisms in the constructor of the data structure, or even some kind of static check or compile-time assertion to make sure everything is as it should be.
But the thing is, if you use a PTM, you only have to call the recovery method of the PTM, one time, regardless of the number of data structure instances you have in your application!
In the case of OneFile, it has null recovery which means there is no recovery method: the PTM will simply continue execution where it left of.

This is yet another thing where PTMs are better than an hand-made durable data structures.

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