Umer Hammad

A cache that doesn't make you wait in line

Apps keep popular data in a cache so they don't have to fetch it every time. When many parts of a program use the same cache at once, the simple design puts one lock on it, so everyone queues for one door.

I wrote one in C++ that splits the cache into 64 independent pieces, each with its own lock. Eight workers almost never collide, so they all get through at once.

Ask a research paper a question

Give it a stack of PDFs and ask a question in plain English. It finds the passages that answer it, writes the answer, and shows you exactly where on which page it came from, so you can check it instead of trusting it.

The question here is a real one, asked of Attention Is All You Need, the paper behind modern AI chatbots.

Building blocks for a rules engine

Open Policy Agent is an open-source rules engine that companies use to decide who is allowed to do what, like “only managers can approve refunds.” Its rules language comes with built-in functions, and the Java version was missing some.

I wrote three of them. Each one has to give exactly the same answer as the original Go version, down to the byte, and the maintainers merged all three.

Who is about to leave, and why

Using records for 7,043 customers of a phone and internet company (a public IBM dataset), I built a model that predicts who is likely to cancel and explains which factors push them there.

The clearest signal is the contract. After five years, only about 3 in 10 month-to-month customers are still around. On two-year contracts, almost everyone is.

Curves traced from the survival chart in the project.