What's dirty memory?

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What's dirty memory?
"Free," "buffer," "swap," "dirty." What does it all mean? If you
said, "something to do with the Summer of '68", you may need a primer
on 'meminfo'.
                                               
The entries in the /proc/meminfo can help explain what's going on with your memory usage, if you know how to read it.
                                               
Example of "cat /proc/meminfo":
                                                root:        total:            used:            free:                  shared:        buffers:        cached:
Mem:          1055760384        1041887232        13873152        0        100417536         711233536
Swap:         1077501952          8540160         1068961792
                                                                                               
                                                MemTotal:                1031016 kB       
MemFree:                13548 kB
MemShared:                0 kB
Buffers:                98064 kB
Cached:                        692320 kB
SwapCached:                2244 kB
Active:                        563112 kB
Inact_dirty:                309584 kB
Inact_clean:                79508 kB
Inact_target:                190440 kB
HighTotal:                130992 kB
HighFree:                1876 kB
LowTotal:                900024 kB
LowFree:                11672 kB
SwapTotal:                1052248 kB
SwapFree:                1043908 kB
Committed_AS:                332340 kB
                                               
                                               
The
information comes in the form of both high-level and low-level
statistics. At the top you see a quick summary of the most common
values people would like to look at. Below you find the individual
values we will discuss. First we will discuss the high-level statistics.
                                                High-Level Statistics
                                                  

MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code)

MemFree: Is sum of LowFree+HighFree (overall stat)

MemShared:  0; is here for compat reasons but always zero.

Buffers: Memory in buffer cache. mostly useless as metric nowadays

Cached: Memory in the pagecache (diskcache) minus SwapCache

SwapCache:
Memory that once was swapped out, is swapped back in but still also is
in the swapfile (if memory is needed it doesn't need to be swapped out
AGAIN because it is already in the swapfile. This saves I/O)
                                                Detailed Level Statistics
VM Statistics
                                               
VM
splits the cache pages into "active" and "inactive" memory. The idea is
that if you need memory and some cache needs to be sacrificed for that,
you take it from inactive since that's expected to be not used. The vm
checks what is used on a regular basis and moves stuff around.
                                               
When
you use memory, the CPU sets a bit in the pagetable and the VM checks
that bit occasionally, and based on that, it can move pages back to
active. And within active there's an order of "longest ago not used"
(roughly, it's a little more complex in reality). The longest-ago used
ones can get moved to inactive. Inactive is split into two in the above
kernel (2.4.18-24.8.0). Some have it three.
                                                  

Active:        Memory that has been used more recently and usually not reclaimed unless absolutely necessary.

Inact_dirty:
Dirty means "might need writing to disk or swap." Takes more work to
free. Examples might be files that have not been written to yet. They
aren't written to memory too soon in order to keep the I/O down. For
instance, if you're writing logs, it might be better to wait until you
have a complete log ready before sending it to disk.

Inact_clean: Assumed to be easily freeable. The kernel will try to keep some clean stuff around always to have a bit of breathing room.

Inact_target:
Just a goal metric the kernel uses for making sure there are enough
inactive pages around. When exceeded, the kernel will not do work to
move pages from active to inactive. A page can also get inactive in a
few other ways, e.g. if you do a long sequential I/O, the kernel
assumes you're not going to use that memory and makes it inactive
preventively. So you can get more inactive pages than the target
because the kernel marks some cache as "more likely to be never used"
and lets it cheat in the "last used" order.
                                                Memory Statistics
                                                  

HighTotal:
is the total amount of memory in the high region. Highmem is all memory
above (approx) 860MB of physical RAM. Kernel uses indirect tricks to
access the high memory region. Data cache can go in this memory region.

LowTotal: The total amount of non-highmem memory.

LowFree:
The amount of free memory of the low memory region. This is the memory
the kernel can address directly. All kernel datastructures need to go
into low memory.

SwapTotal: Total amount of physical swap memory.

SwapFree: Total amount of swap memory free.

Committed_AS:
An estimate of how much RAM you would need to make a 99.99% guarantee
that there never is OOM (out of memory) for this workload. Normally the
kernel will overcommit memory. That means, say you do a 1GB malloc,
nothing happens, really. Only when you start USING that malloc memory
you will get real memory on demand, and just as much as you use. So you
sort of take a mortgage and hope the bank doesn't go bust. Other cases
might include when you mmap a file that's shared only when you write to
it and you get a private copy of that data. While it normally is shared
between processes. The Committed_AS is a guesstimate of how much
RAM/swap you would need worst-case.
               
               
               

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