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http://engineering.tumblr.com/post/48701285213/open-source-memcache-top

by hamstah • 13 years ago

67 11 13 years ago

engineering.tumblr.com

n1c• 13 years ago

Larger screenshot from the Github page: https://raw.github.com/wiki/bmatheny/memkeys/misc/screenshot...

pestaa• 13 years ago

The only useful line to me is the second bottom one. It says the packet loss is more than 4% as opposed to less than 2% the article claims.

Still a nice tool, though.

lobo_tuerto• 13 years ago

Maybe the traffic is > 1Gb/s in this case.

bmatheny2• 13 years ago

This screenshot was taken on my dev box which has plenty of other activity happening. On a production memcache box, seeing 1Gb/s, I see ~2% packet loss.

jbert• 13 years ago

Does dropping packets matter if you're looking for hot keys?

As long as you manage to be fair in what you drop, surely all you really need is a representative sample (even 1-in-100 or whatever would do).

corresation• 13 years ago

This was my sentiment as well. Further I'm not quite sure how this works -- you run one of these instances on every single memcache instance machine, using packet capture? No mechanism to work across a cluster?

It seems like something one would make a patch for memcache itself (where injecting such metrics seems quite simple).

mmuro• 13 years ago

I guess when you get up to the scale of Tumblr, you need any measure of performance. I wish I knew what was going on there. Anyone care to elaborate?

nasalgoat• 13 years ago

At Tumblr or in the supplied image?

If the latter, it's a list of memcache key IDs, ordered by hit count.

alekseyk• 13 years ago

Cool tool, a bit of a pointless jab at Etsy.

I don't know how it queries up Memcached but it's probably a good idea to see how it affects performance of it before running it against a production cache pool.

Just a thought.

crescentfresh• 13 years ago

Note: going solely off the two screenshots (mctop: http://etsycodeascraft.files.wordpress.com/2012/12/mctop.jpg , memkeys: https://raw.github.com/wiki/bmatheny/memkeys/misc/screenshot...), it's not clear why tumblr didn't fix the stated packet loss issue in mctop instead.

bmatheny2• 13 years ago

Since I wrote memkeys, maybe I can clarify a few things.

First, dropping packets matters. If you see only 30-40% of your traffic you can't guarantee that you have enough data to know what your hot keys actually are. This is especially true when you are interested in (for instance) sorting keys by bandwidth usage. You might have a key that gets half as many hits as the hottest key but is 4x the size and causing network link saturation. In this case, depending on how much data you're able to capture, you may or may not even see this data point. Also, the follow-up comment from corresation about patching memcache doesn't make sense to me.

Second, this was no 'jab' at etsy. I know the etsy guys incredibly well and we're all friends. We've collaborated on work in more than one occasion. The jab comment seems like unnecessary speculation. The comment about seeing how memkeys affects performance is of course spot on. In this case, one thread will peg a CPU core for packet capture but besides that will not be CPU intensive. Since it uses packet capture, memkeys doesn't actually interact with memcached directly so the impact should be minimal. We used it at Tumblr.

Third, fixing the packet loss issue in mctop wasn't feasible as the problem is with ruby-pcap not with mctop. Additionally, while Tumblr has plenty of ruby code in production we don't generally use it for building 'real-time' applications. There are better languages for the job.

I built memkeys because it solved a problem we had, and was fun. That's it.

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