New Safe Memory Reclamation feature in UMA

Jeff Roberson jroberson at jroberson.net

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Hello Folks,

I want to make the larger community aware of a substantial feature coming 
to UMA soon.  The review is at <a href="https://reviews.freebsd.org/D22586">https://reviews.freebsd.org/D22586</a> along 
with some perf results.

SMR is a technique that allows for various types of lockless 
synchronization by eliminating use-after-free hazards.  This is in the 
same family as RCU/QSBR/EPOCH/Parsec.  There is quite a lot of material 
available on the uses of these algorithms.

Most of these algorithms suffer from holding on to freed memory for a 
relatively long period of time and reclaiming it when it's cache-cold. 
This also creates quite a lot of resource starvation edge cases.  This is 
evident by the amount of code in RCU on linux intended to work around 
these issues.  These algorithms are generally best with a small write/free 
workload and a very heavy read workload.

For the virtual memory system I needed something that could sustain 
relatively rapid frees.  I have ended up with a scheme that integrates 
with the allocator and uses a novel epoch/version tracking mechanism. 
The pair of these gives me 3x faster performance with 1/20th the memory 
overhead of our existing epoch implementation in my obviously contrived 
perf test.  I do not want to imply that if we replaced the network epoch 
with uma smr the network stack would go 3x faster.  I do think there may 
be benefits there especially for things with high turnover like pcbs.  I 
do not yet have support for sleepable sections so there is a lot of 
technical space between here and there.

There is a lot of information in the review and comments in the code.  I 
will be validating on weaker memory ordering architectures.  I need to 
write a man page.  I would like to find a snappy name to avoid confusion 
with other algorithms.  If anyone has suggestions I am open to it.

Thanks,
Jeff

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