Saturday, August 23, 2014

Memory access latencies

Once, I saw a table in which all the memory latencies are scaled in such a way that CPU cycle is defined to be 1 second, and then L1 cache latency is several seconds, L2 cache even more, and so on up to SCSI commands timeout and system reboot. This was very interesting because I have much better developed sense for seconds and higher time units that for nanoseconds, microseconds, etc. Few days ago I remembered that table and I wanted to see it again, but couldn't find it.  This was from some book I couldn't remember the name. So, I started to google for it, and finally, after an hour or so of googling, I managed to find this picture. It turns out that this was from the book Systems performance written by Brendan Gregg. So, I decided to replicate it here for a future reference:

Table 2.2: Example Time Scale of System Latencies
Event Latency Scaled
1 CPU Cycle 0.3 ns 1 s
Level 1 cache access 0.9 ns 3 s
Level 2 cache access 2.8 ns 9 s
Level 3 cache access 12.9 ns 43 s
Main memory access (DRAM, from CPU) 120 ns 6 min
Solid-state disk I/O (flash memory) 50 - 150 us 2-6 days
Rotational disk I/O 1-10 ms 1-12 months
Internet: San Francisco to New York 40 ms 4 years
Internet: San Francisco to United Kingdom 81 ms 8 years
Internet: San Francisco to Australia 183 ms 19 years
TCP packet retransmit 1-3 s 105-317 years
OS virtualization system reboot 4 s 423 years
SCSI command timeout 30 s 3 millennia
Hardware (HW) virtualization system reboot 40 s 4 millennia
Physical system reboot 5 min 32 millennia

It's actually impressive how fast CPU is with respect to other components. It is also very good argument for multitasking, i.e. assigning CPU to some other task while waiting for, e.g. disk, or something from the network.

One additional impressive thing is written below the table in the book. Namely, if you multiply CPU cycle with speed of light (c) you can see that the light can travel only 0.5m while CPU does one instruction. That's really impressive. :)

That's it for this post. For the end, while I was searching for this table, I stumbled on some additional interesting links:

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scientist, consultant, security specialist, networking guy, system administrator, philosopher ;)

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