Paper notes: Anti-Caching

Anti-Caching: A New Approach to Database Management System Architecture

DeBrabant et. al., VLDB 2013

The big idea: Traditional databases typically rely on the OS page cache to bring hot tuples into memory and keep them there. This suffers from a number of problems:

  • No control over granularity of caching or eviction (so keeping a tuple in memory might keep all the tuples in its page as well, even though there’s not necessarily a usage correlation between them)
  • No control over when fetches are performed (fetches are typically slow, and transactions may hold onto locks or latches while the access is being made)
  • Duplication of resources – tuples can occupy both disk blocks and memory pages.

Instead, this paper proposes a DB-controlled mechanism for tuple caching and eviction called anti-caching. The idea is that the DB chooses exactly what to evict and when. The ‘anti’ aspect arises when you consider that the disk is now the place to store recently unused tuples, not the source of ground truth for the entire database. The disk, in fact, can’t easily store the tuples that are in memory because, as we shall see, the anti-caching mechanism may choose to write tuples into arbitrary blocks upon eviction, which will not correspond to the pages that are already on disk, giving rise to a complex rewrite / compaction problem on eviction. The benefits are realised partly in IO: only tuples that are cold are evicted (rather than those that are unluckily sitting in the same page), and fetches and evictions may be batched.

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Paper notes: Stream Processing at Google with Millwheel

MillWheel: Fault-Tolerant Stream Processing at Internet Scale

Akidau et. al., VLDB 2013

The big idea: Streaming computations at scale are nothing new. Millwheel is a standard DAG stream processor, but one that runs at ‘Google’ scale. This paper really answers the following questions: what guarantees should be made about delivery and fault-tolerance to support most common use cases cheaply? What optimisations become available if you choose these guarantees carefully?

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Paper notes: DB2 with BLU Acceleration

DB2 with BLU Acceleration: So Much More than Just a Column Store

Raman et. al., VLDB 2013

The big idea: IBM’s venerable DB2 technology was based on traditional row-based technology. By moving to a columnar execution engine, and crucially then by taking full advantage of the optimisations that columnar formats allow, the ‘BLU Acceleration’ project was able to improve read-mostly BI workloads by a 10 to 50 times speed-up.

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Étale cohomology

The second in an extremely irregular series of posts made on behalf of my father, who has spent much of his retirement so far doing very hard mathematics. What is attached here is the essay he wrote for the Part III of the Cambridge Mathematical Tripos, a one year taught course. The subject is the Étale cohomology.

Says my Dad: “I am afraid that I have been lured away from the translation of SGA 4.5 for some time by the attraction of working on Wolfgang Krull’s report on “Idealtheorie” from 1935 (again I am not aware of an English version anywhere) which is yet another important classic. However during a year at Cambridge I did write an essay as a very basic introduction to Étale Cohomology which was based on the first part of SGA 4.5. So with the usual imprecation of caveat lector, here it is as a temporising partial substitute should any other beginner be interested.”

Here’s part of the introduction:

This essay has been written as part of the one year Certificate of Advanced Study in Mathematics (CASM) course at Cambridge University which coincides with Part III of the Mathematical Tripos. The starting point is, of necessity, roughly that reached in the lectures which in this particular year did not include much in the way of schemes and sheaves, nor, in the case of the author, much in the way of algebraic number theory.
Thus the frontiers of the subject can safely rest undisturbed by the contents of this essay. Rather it has been written with a reader in mind corresponding roughly to the author at the start of the enterprise. That is someone who is interested to find out what all the fuss was with the French algebraic geometers in the 1960s but is in need of some fairly elementary background to map out the abstractions involved and with any luck to avoid drowning in the “rising sea”.

And here’s the essay itself!