disk IO, memory, or CPU - What is slowing me down?
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Impossible to say, totally depends on your workload. For example, a workload that needs very little RAM and CPU power, but reads or writes massive amounts of data to an I/O-subsystem will of course mostly be influnced by the speed of the I/O-subsystem. If you have a workload that needs a certain amount of RAM not having that amount of RAM would cause the machine to swap, so here RAM will have the most influence (and the I/O-subsystem will have a smaller influence, since swapping speed depends on that). If you have a workload that needs massive amounts of CPU power, but not that much RAM or I/O speed (like high quality video encoding) of course CPU will be the bottleneck.
It depends on what you are doing with your computer.
For most people, the internet connection is the "bottleneck" component causing most slow down.
It may be a matter of definition:
L2 cache misses may be a significant portion of delay. If so, does that mean your ram is too slow (that delay would be less if ram were faster) or does it mean your L2 cache (part of the CPU) is too small?
Similarly for file cache misses. Do you have too little ram causing more file cache misses, or do you have too slow a hard drive, so file cache misses take longer?
There are no right answers to the above, it is just point of view.
Tobi's right, of course. What you are most slowed down by, is what you've not got enough of. And, when you think about it, that hardly says anything; you are slowed down by it, and that's what defines the amount that you have of it as not enough, whether it be disk or network bandwidth, cpu, or RAM (and ram could be amount of ram or it could be bandwidth to ram).
That said, if you see a Linux computer struggling, the chances are quite strong that it is swapping and that the best cure for that would be more RAM. Part of the reason for that is that, particularly for computers that were provisioned/bought/specified some time ago, the requirements have probably increased and the amount of RAM has not kept pace.
Most of the time, though, the computer spends waiting for the squidgy thing to press another key, and the squidgy things are quite happy about that situation. When the revolution comes and the computers can have their squidgy things upgraded because they don't press the keys quickly enough, then you'll see a change, and I doubt that you'll like it!
and don't forget to monitor at a reasonably high frequency,on the order of 5-10 seconds, not 5 to 10 minutes like far too many people do.
and then PLOT it as there will be too much to just look at