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Last active November 29, 2023 08:30
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Latency Numbers Every Programmer Should Know
Latency Comparison Numbers (~2012)
L1 cache reference 0.5 ns
Branch mispredict 5 ns
L2 cache reference 7 ns 14x L1 cache
Mutex lock/unlock 25 ns
Main memory reference 100 ns 20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy 3,000 ns 3 us
Send 1K bytes over 1 Gbps network 10,000 ns 10 us
Read 4K randomly from SSD* 150,000 ns 150 us ~1GB/sec SSD
Read 1 MB sequentially from memory 250,000 ns 250 us
Round trip within same datacenter 500,000 ns 500 us
Read 1 MB sequentially from SSD* 1,000,000 ns 1,000 us 1 ms ~1GB/sec SSD, 4X memory
Disk seek 10,000,000 ns 10,000 us 10 ms 20x datacenter roundtrip
Read 1 MB sequentially from disk 20,000,000 ns 20,000 us 20 ms 80x memory, 20X SSD
Send packet CA->Netherlands->CA 150,000,000 ns 150,000 us 150 ms
1 ns = 10^-9 seconds
1 us = 10^-6 seconds = 1,000 ns
1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns
By Jeff Dean:
Originally by Peter Norvig:
'Humanized' comparison:
Visual comparison chart:
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Yrwein commented Oct 4, 2022

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Send 1K bytes over 1 Gbps network 10,000 ns 10 us

this seems misleading, since in common networking terminology 1 Gbps refers to throughput ("size of the pipe"), but this list is about "latency," which is generally independent of throughput - it takes the same amount of time to send 1K bytes over a 1 Mbps network and a 1 Gbps network

A better description of this measure sounds like "bit rate," or more specifically the "data signaling rate" (DSR) over some communications medium (like fiber). This also avoids the ambiguity of "over" the network (how much distance?) because DSR measures "aggregate rate at which data passes a point" instead of a segment.

Using this definition (which I just learned a minute ago), perhaps a better label would be:

- Send 1K bytes over 1 Gbps network       10,000   ns       10 us
+ Transfer 1K bytes over a point on a 1 Gbps fiber channel       10,000   ns       10 us

🤷 (also, I didn't check if the math is consistent with this labeling, but I did pull "fiber channel" from the table on the DSR wiki page)

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nking commented Jun 8, 2023

Thanks for sharing your updates.

You could consider adding a context switch for threads right under disk seek:
computer context switches: 1e7 ns

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