Paper Read- Twizzler: A Data Centric OS

The current progamming model is predicated upon the fact that the fastest memory access is via the RAM, which is volatile. Due to the violatile nature of RAM, programming systems have had to guard against crashes. Especially data centric systems like databases. Power loss cannot be an excuse for someone losing money in their bank account. But these durability settings negatively affect performace. PostgresSQL in fact has turning off durability as a performace tip!

NVDIMM represents a change in this paradigm. NVDIMM is a type of non-volatile storage that sits in the DIMM slots in the motherboard. It's latency sits squarely between an NVME SSD and DRAM. But the key takeway is that we now have access to a large amount of persistent memory at relatively low latency!

Daniel Bittman, et al., in their paper argue that software needs to be rebuilt from the ground up keeping in mind that persistent RAM exsists. Kernel's generally need to seperate volatile and non-violatle memory, by maintaining a copy of the data in RAM and flushing to disk as necessary. Managing these copies of data incurs a performance cost that does not need be borne. Kernels need to allow direct manipulation of RAM for NVDIMMs to be used to thier full potential.

Twizzler is an OS that explores this idea. The kernel gets out of the way when manipulating data. They do so by providing primitives to directly read and write data via a concept called persistent pointers. Persistent pointers are a way to directly address data in RAM. They can be named, and the OS provides a way to use a custom name resolver, so that a name points to a particular piece of data. An OS that allows for direct manipulation of persistent data and has persistent pointers is what the paper calls a data centric OS.

One of the intersting consequences of this approach is that the process abstraction is no longer needed. Processes use virtual address space along with a security role(permissions like read, write and read-write) and state in kernel (a proces table, page table etc) to maintain isolation. However, now that the thread can access data without asking the kernel, security and virtual address's are decoupled.

In Twizzler OS, a single mapped piece of data is called an Object with it's own Object ID. Objects can vary in size, and the access control is directly defined by the memory managemnt unit. An object can be a single node of a B-tree, or the entire B-tree itself! The choice is down to the programmer. To allow programs to directly access data, there is an object called the "view", which maps the data onto the DIMM using a virtual addrees. Adding new entries is free, without kernel intervention, but deleting or changing the entry is not. To allow for persistent pointers, another object called the Foriegn Object Table exists. This handles the name resolution and binding, and also has permission request flags.

Another interesting concept is that security in Twizzler is late bound. If you ask for a Read-Write permisson on a read only object, but only read the data in the program, no error is thrown! Threads in Twizzler run in a securit context, which stores the access rights the thread has for objects.

The team implemented a new kernel, and also built a translation layer to map musl (libc) to their library. They then ported SQLLite to Twizzler and compared the performace. SQLLite is 4x times fater than any competiting platform! The work done here is really impressive.

Also, a special shout out to logboi, which is the name of the logging daemon they built!

This paper reminds us that the Operating Systems software that we have today are built to combat the restrictions of the hardware they had to deal with. Abstractions like processes were bulit because you had to enable multi-tasking. You had to implement a control to ensure one program does not wipe another program's data without permission. You needed to be able to efficiently manage the memory a process had access to because only RAM provided a desireable user experience. With advancing hardware innovations, these restrictions are no longer at play.