Skip to main content

Documentation Index

Fetch the complete documentation index at: https://mintlify.com/donnemartin/system-design-primer/llms.txt

Use this file to discover all available pages before exploring further.

Understanding latency numbers is crucial for making informed decisions in system design. These numbers help you estimate performance characteristics and identify potential bottlenecks.

​
Latency Comparison Numbers

Latency Comparison Numbers
--------------------------
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            10,000   ns       10 us
Send 1 KB bytes over 1 Gbps network     10,000   ns       10 us
Read 4 KB 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
HDD seek                            10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
Read 1 MB sequentially from 1 Gbps  10,000,000   ns   10,000 us   10 ms  40x memory, 10X SSD
Read 1 MB sequentially from HDD     30,000,000   ns   30,000 us   30 ms 120x memory, 30X SSD
Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms

​
Time Unit Conversions

Notes
-----
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

​
Handy Metrics

Based on the numbers above, here are some useful throughput metrics:
  • Read sequentially from HDD at 30 MB/s
  • Read sequentially from 1 Gbps Ethernet at 100 MB/s
  • Read sequentially from SSD at 1 GB/s
  • Read sequentially from main memory at 4 GB/s
  • 6-7 world-wide round trips per second
  • 2,000 round trips per second within a data center

​
Latency Numbers Visualized

Latency visualization

​
Key Takeaways

  1. Memory is fast - Main memory is significantly faster than disk storage
  2. Network is slow - Even within the same datacenter, network calls add significant latency
  3. Sequential access is faster - Reading sequentially is much faster than random access
  4. SSD vs HDD - SSDs are approximately 30x faster than HDDs for sequential reads
  5. Geographic latency - Cross-continental network calls take 150ms+

​
Further Reading