Replacing the cache replacement algorithm in memcached

In this post we delve into a reworking of memcached’s Least Recently Used (LRU) algorithm which was made default when 1.5.0 was released. Most of these features have been available via the “-o modern” switch for years. The 1.5.x series has enabled them all to work in concert to reduce RAM requirements.

When memcached was first deployed, it was typically co-located on backend web servers, using spare RAM and CPU cycles. It was important that it stay light on CPU usage while being fast; otherwise it would affect the performance of the application it was attempting to improve.

Over time, the deployment style has changed. There are frequently fewer dedicated nodes with more RAM, but spare CPU. On top of this web requests can fetch dozens to hundreds of objects at once, with the request latency having a greater overall impact.

This post is focused on the efforts to reduce the number of expired items wasting cache space, general LRU improvements, as well as latency consistency.

https://memcached.org/blog/modern-lru/

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Caching at Reddit

Performance matters. One of the first tools we as developers reach for when looking to get more performance out of a system is caching. As Reddit has grown in users and response times have improved, the amount of caching has grown to be quite large as well.

In this post we’ll talk about some of the nuts-and-bolts numbers of Reddit’s caching infrastructure—the number of instances, size of instances, and overall throughput. We hope that sharing this information may help others gauge what type of performance and sizing they can expect when building similar clusters. At the very least, we hope you’ll find it interesting to see a bit more about how Reddit works under the hood.

We’ll also go over the Reddit-specific type of work our caches do, how we use mcrouter to manage our caches more effectively, and the custom monitoring (MemcachedSlabCollector and mcsauna) we’ve written to help us understand what’s going on behind the scenes. We’ll also talk about some of the more subtle issues that we’ve run into when deploying changes to our caches.

Caching at Reddit

Introducing mcrouter: A memcached protocol router for scaling memcached deployments

“Since any client that wants to talk to memcached can already speak the standard ASCII memcached protocol, we use that as the common API and enter the picture silently. To a client, mcrouter looks like a memcached server. To a server, mcrouter looks like a normal memcached client. But mcrouter’s feature-rich configurability makes it more than a simple proxy…”

https://code.facebook.com/posts/296442737213493/introducing-mcrouter-a-memcached-protocol-router-for-scaling-memcached-deployments/