7. Cache - iPhone, Android and Desktop

Desktop and Smartphone

The table below shows the typical memory and caches used within a desktop \ laptop CPU. In this case an Intel 4 core CPU.

All cache memory at the end of the day is a set of logic gates, so it is useful to appreciate where those gates actually reside - hence the 'physical location' column.

'Latency' is the number of clock cycles it takes to make a data item available. It starts with the very fast, but smallest Level 1 cache (4 cycles) that is closest to the processing core, then each cache is slower but more remote from the core itself. The table also includes the external RAM which takes about 117 cycles and the really slow virtual memory of the swap file stored on the hard disk.

Memory	Size	Latency	Physical Location
L1 cache	32KB	4 cycles	within each core
L2 cache	256KB	11 cycles	beside each core
L3 cache	6 MB	~ 21 cycles	shared with all cores
L4 E-cache	128MB	~ 58 cycles	separate chip
RAM	4GB +	~ 117 cycles	motherboard ram
Swap File	100GB +	~ 10,000 cycles	hard disk

Table: Intel Core i*4 arrangement

And mobile phones also use the same idea - this is the memory arrangement of the Apple iPhone 6

Memory	Size	Latency	Physical Location
L1 cache	64KB	4 cycles	inside each core
L2 cache	1MB	~20 cycles	next to the 2 cores
L3 cache	4MB	~ 107 cycles	next to the memory controller
RAM	1GB	~ 261 cycles	external chips

Table: iPhone 6 memory arrangement

So even though a powerful desktop computer seems to have no relationship to the smartphone in your hand, both are using the same ideas in trying to make the maximum use of their powerful CPU despite the memory bottlenecks in the way.

Challenge see if you can find out one extra fact on this topic that we haven't already told you

Click on this link: Code optimisation with cache