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Report from the Adhoc Event 23 April

24 Apr 2020

I attended a remote virtual event organized by Paul Khuong, Samy Al Bahra and Jessica Natoli. Four short 15 minutes talks followed by Q&A. Great topics, professional, pedagogic speakers; and the format - short talks with recommended reading - before attending was great. I only wished that I had more time to prepare. A big thank you to Paul, Samy, Jessica, and all four presenters! I hope there will be more events!

Making Safe Memory Reclamation a Feature Of Your Memory Allocator

Jeff Roberson talked about how he added a novel memory reclamation scheme to the Free BSD slab allocator. Lots of new things for me here. I’ve heard about slab allocators before but I’ve never taken the time to read up on how they work. I knew that you’re supposed to keep objects of the same type in bins. What was new to me was that you separate the malloc/free part from the construct/destruct of the objects. You allocate a slab with a bunch of objects and call the constructor function on them. Then you hand the objects out as the clients need them. But when you get an object back, you keep it around in a “base-constructed” state. So when it’s needed again you can just hand it out.

Slab allocators are a neat idea and I think I could grasp it, but then there’s memory reclamation. Phew! That was way harder. I had never heard of memory reclamation. As I understand it, for lock-free data structures, we need a way to know when an object is not used anymore. And we need to figure it out without blocking. We can use reference counting or we can use hazard pointers or use quiescent-state-based grace periods (QBSR) or epoch grace periods (EBR). Each of those ideas was new to me, but they’ve existed for a while and are used in different forms for non-blocking data structures.

But Jeff has figured out something new. He calls it Global Unbounded Sequence (GUS). It shares some similarities with the Epoch-based approach but allows a larger range of sequence numbers. One thing I had a hard time figuring out when reading the background material was how the actual detection of memory ready to be reclaimed happens. Do we need a separate thread that scans the list of objects or can we do it upon every read/write operation?

Jeff describes how GUS can do the checks per slab instead of per object which increases performance. There’s one slab in the per-CPU cache for allocations and one slab for frees. When the frees slabs fill up, it’s passed to a free list and stamped with the lowest sequence number visible at that time. When the slab reaches the front of that list, the allocator checks if it can be reused for allocations by comparing its sequence number to the global sequence number. I so which that I knew more about memory reclamations, but it sounds like a great idea to me at least!

A 15-minute History of Task Isolation on Linux

Matt Flemming described how task isolation has improved in the kernel. When I first saw the title of the talk I thought Matt was going to talk about process isolation - how every resource can be virtualized with cgroups and such, but that was not the case.

What he and others have tried to achieve is to minimize interruptions - a process should run as if it was the only process on a CPU, preferable it should be the only process. When I heard that I thought of RTOSs and userspace networking. And yup, that’s the kind of users they’re targetting.

They want to get rid of preemption, interrupts, timers, and RCU callbacks. I’m not quite sure how RCU callbacks are used in the kernel.

When I’m benchmarking I use taskset for binding a process to certain cores. That is done through the sched_setaffinity call. I’ve also used heard of people using cset. I wonder if it uses the same mechanism? I think I’ve read somewhere that it is supposed to block out interrupts as well. Matt talks about the isolcpu kernel command-line parameter. I need to read up on that one.

From version 3.10 there is a nohz_full but it still caused a tick every second. But in version 4.17, that tick is disabled. But Perf events and timers can still cause some ticks. There is a task isolation mode that is in the process of being upstreamed.

I wonder what the future use cases for this will be? Is it possible that more services besides networking may move into user-space and use isolated CPUs for running their workloads?

Why Mmap is Faster Than System Calls

Alexandra Feodorova has written a storage engine for modern fast SSD devices. She described how due to decreasing SSD wait times, the filesystem code has become a bottleneck. There is newer Linux file system but they haven’t been tested much yet. I believe the standards for filesystem testing is higher than for many other components of a software stack.

So instead of rewriting kernel parts, she and her colleagues wrote a new storage engine for MongoDB. They used mmap since that means less system call overhead, and memcpy from a mmapped buffer is substantially faster than read’s copy_to_user. That is because the kernel can’t use vector instructions. Todd Lipcon said that newer CPUs have an enhanced REP MOVSB (ERMSB) which should perform about as good.

For the implementation of the MongoDB storage engine, they split the threads into writers and readers. A write was anyone writing beyond EOF, calling fallocate, or ftruncate. When a writer is active, I/O is routed to system calls which are properly synchronized.

The write pseudo-code is executed before a write op

wait:
	while mmap_resizing
		spin_backoff(...)
	if cas(mmap_resizing, 1, ...)
		goto wait
	while mmap_use_count
		spin_backoff(...)
	do_write(...)
	unmap(file)
	remap(file, file)
	mmap_resizing = 0

And here’s read (something similar needs to be done by writers too for checking if they can use mmap)

atomic_add(mmap_use_count, 1)
if mmap_resizing
	atomic_decr(mmap_use_count, 1)
	read_syscall(...)
else
	memcpy(dst_buf, mapped_buffer, ...)
	atomic_decr(mmap_use_count, 1)

jitmap, an execution engine for bitmap expressions

Francois Saint-Jacques was presenting on an interesting subject, but I didn’t have the time to do the background reading or watch the presentation. I’ll add a small summary here, once I’ve watched the recording.