[nneonneo]

The title is misleading. 77% slower sounds like the system calls take 1.77x the time on EC2. In fact, the results indicate that the normal calls are 77% faster - in other words, EC2 gettimeofday and clock_gettime calls take nearly 4.5x longer to run on EC2 than they do on ordinary systems.

This is a big speed hit. Some programs can use gettimeofday extremely frequently - for example, many programs call timing functions when logging, performing sleeps, or even constantly during computations (e.g. to implement a poor-man's computation timeout).

The article suggests changing the time source to tsc as a workaround, but also warns that it could cause unwanted backwards time warps - making it dangerous to use in production. I'd be curious to hear from those who are using it in production how they avoided the "time warp" issue.

[klodolph]

77% faster is not correct either. "Speed" would probably by ops/s.

4.5x longer = 350% slower.

[stouset]

Even this is confusing as hell.

Just say the native calls take 22% of the time they do on EC2. Or that the EC2 calls take 450% of the time of their native counterparts.

"Faster" and "slower" when going with percentages are ripe with confusion. Please don't use them.

[klodolph]

I can't agree. Speed is usually units/time, and everyone knows that 100 mph is 2x as fast as 50 mph, or 100% faster.

[tropo]

This is what decibels are for.

[derefr]

> Some programs can use gettimeofday extremely frequently

This is what's usually considered the "root cause" of this problem, though. It's easy enough, if it's your own program, to wrap the OS time APIs to cache the evaluated timestamp for one event-loop (or for a given length of realtime by checking with the TSC.) Most modern interpreters/VM runtimes also do this.

[kzrdude]

Why cache it when the vdso has already solved the problem? Seems best to not stack another mitigation on top of it.

[derefr]

Because you're writing portable code and not every (or even most) systems do said caching.

[lathiat]

Yeah like the php Xdebug extension. In 5.3 at least even with it just loaded and nothing enabled it called gettimeofday 1000s of times and would add seconds to web app render times for me (also on Xen with slow gettimeofday)