How fast is cache or memory compared to hard drive?
Nov 16, 2021 20:34:32 GMT -5
Carl Gundel, Brandon Parker, and 2 more like this
Post by Chris Iverson on Nov 16, 2021 20:34:32 GMT -5
So, I've seen a few different versions of this, but it all stems from the same idea.
We sometimes have a poor understanding of how relatively slow different parts of our computers can be to retrieve data. It's all less than a second, so accessing memory vs accessing a hard drive isn't that much different, right?
Well, it's actually differences of orders of magnitude; we just don't notice it because, as said, it's all less than a second to us.
So, to get a better idea of the various time differences for various parts of your computer, we can scale those times up to human time, instead, making 1 nanosecond equal to 1 second, and then scaling the rest of the times up from there, to put it on a human timescale.
Data is from here: https://gist.github.com/hellerbarde/2843375
So, here's a list of some data moving operations, and approximate actual times they take.
So, we can see that, quite literally, reading from a spinning disk hard drive is millions of times slower than reading data from memory. But even that scale still doesn't mean much.
So let's convert that as shown above, and shown what the time scales would be then.
If we scale up so that 1 ns = 1 s, then we get:
That's a much better visualization of the differences of time scale for the various operations, I think.
We sometimes have a poor understanding of how relatively slow different parts of our computers can be to retrieve data. It's all less than a second, so accessing memory vs accessing a hard drive isn't that much different, right?
Well, it's actually differences of orders of magnitude; we just don't notice it because, as said, it's all less than a second to us.
So, to get a better idea of the various time differences for various parts of your computer, we can scale those times up to human time, instead, making 1 nanosecond equal to 1 second, and then scaling the rest of the times up from there, to put it on a human timescale.
Data is from here: https://gist.github.com/hellerbarde/2843375
So, here's a list of some data moving operations, and approximate actual times they take.
L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns
L2 cache reference ........................... 7 ns
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns
Compress 1K bytes with Zippy ............. 3,000 ns = 3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns = 20 µs
SSD random read ........................ 150,000 ns = 150 µs
Read 1 MB sequentially from memory ..... 250,000 ns = 250 µs
Round trip within same datacenter ...... 500,000 ns = 0.5 ms
Read 1 MB sequentially from SSD* ..... 1,000,000 ns = 1 ms
Disk seek ........................... 10,000,000 ns = 10 ms
Read 1 MB sequentially from disk .... 20,000,000 ns = 20 ms
Send packet CA->Netherlands->CA .... 150,000,000 ns = 150 msSo, we can see that, quite literally, reading from a spinning disk hard drive is millions of times slower than reading data from memory. But even that scale still doesn't mean much.
So let's convert that as shown above, and shown what the time scales would be then.
If we scale up so that 1 ns = 1 s, then we get:
L1 cache reference 0.5 s One heart beat (0.5 s)
Branch mispredict 5 s Yawn
L2 cache reference 7 s Long yawn
Mutex lock/unlock 25 s Making a coffee
Main memory reference 100 s Brushing your teeth
Compress 1K bytes with Zippy 50 min One episode of a TV show (including ad breaks)
Send 2K bytes over 1 Gbps network 5.5 hr From lunch to end of work day
SSD random read 1.7 days A normal weekend
Read 1 MB sequentially from memory 2.9 days A long weekend
Round trip within same datacenter 5.8 days A medium vacation
Read 1 MB sequentially from SSD 11.6 days Waiting for almost 2 weeks for a delivery
Disk seek 16.5 weeks A semester in university
Read 1 MB sequentially from disk 7.8 months Almost producing a new human being
The above 2 together 1 year
Send packet CA->Netherlands->CA 4.8 years Average time it takes to complete a bachelor's degreeThat's a much better visualization of the differences of time scale for the various operations, I think.
