The Harvester gathers a collection of memory samples.
These counters are not always accurate. It’s possible for the sum of the various page types (free_bytes, mmu_overhead_bytes, free_heap_bytes, and more) to temporarily add up to more than the total memory. In the rare cases where the values don't add up accurately, the differences should only one or two memory pages.
The counts are tracked by incrementing/decrementing atomic integers when a page transitions from one state to another (for example, from free_bytes to free_heap_bytes). The relaxed accuracy allows for higher system performance. This can be adjusted in the future, but so far there's been no call to make the trade for that extra tiny bit of accuracy.
The path to each sample includes memory and the sample name. For example, memory:free_bytes.
Graph data collected by the Harvester along with timestamp and a Dockyard path is called a sample. The following sections describe the samples collected.
This data is often tracked in pages. So values will change by several KB at a time.
Device memory refers to the memory within the machine. It's not specific to any process or the kernel.
The total physical memory available to the machine.
The bytes within device_total_bytes that are unallocated.
This memory is related to the kernel rather than any user process or ipc.
The total kernel bytes as reported by page state counter.
The bytes within kernel_total_bytes that are unallocated.
The amount of memory reserved by and mapped into the kernel for reasons not reported elsewhere. Typically for read-only data like the RAM disk and kernel image, and for early-boot dynamic memory.
These group memory used by category, with a catch-all ‘other’ category for miscellaneous memory that doesn't fit in another category.
The number of bytes used for Virtual Memory Objects. Ownership of a VMO may be transferred between processes.
Tracking the memory state also requires memory. This is the number of bytes of overhead used for tracking page tables.
Current amount of memory used for inter-process communication. Currently this reflects the memory used for Zircon channels, but in the future it may include memory for sockets and fifos.
Memory that is in use but not tracked as kernel, user, ipc, and so on.