Memory Fragmentation

Introduction

Version 3.2.1 of collectl introduces support for /proc/buddyinfo, which shows the distribution of memory fragments where the size of each fragment is a power of 2 pages. The memory is categorized by node, depending on the system architecture and then subcategorized by the type of memory, referred as a zone. For example, the /proc/buddyinfo might look like:

Node 0, zone      DMA      5      5      3      4      2      4      3      1      0      0      0
Node 0, zone    DMA32     79     61      4     12      0      0      0      2      0      1      0
Node 1, zone    DMA32    134     57     27     60      0      0      1      1      0      1      0
Node 2, zone   Normal    865    357     37      1      6      0      2      1      0      1      0
Node 3, zone   Normal    651     47     19     10      1      1      1      1      0      1      0

Running collectl with -sb --verbose would produce a single line of output that shows he totals of each column. For example, taking 1 second samples with timestamps included:

collectl -sb --verbose -oT
# MEMORY FRAGMENTATION SUMMARY (4K pages)
#                1       2       4       8      16      32      64     128     256     512    1024
16:11:26      1296     483     157      85       9       5       7       6       0       4       0
16:11:27      1354     485     163      87       9       5       7       6       0       4       0
16:11:28      1395     480     165      89       9       5       7       6       0       4       0

And in detail mode we see one line per entry, again with timestamps:

collectl -sB -oT
# MEMORY FRAGMENTATION (4K pages)
#         Node    Zone        1       2       4       8      16      32      64     128     256     512    1024
16:13:33     0     DMA        5       5       3       4       2       4       3       1       0       0       0
16:13:33     0   DMA32      175      97       3      12       0       0       0       2       0       1       0
16:13:33     1   DMA32      933     389      60      68       0       0       1       1       0       1       0
16:13:33     2  Normal        0       1       8       1       6       0       2       1       0       1       0
16:13:33     3  Normal        1       2      57      10       1       1       1       1       0       1       0

Where things get interesting is in brief mode. The challege here is to show the maximum amount of information in the least amount of space, thus allowing you to look at other information on the same line as well. To better understand the methodology chosen for this, think in terms of base 36. However, instead of mapping each character to a number from 0 to 35, we're going to do something very different. Since we're mainly interested in seeing what's happening as the numbers of fragments shrink:

map the 0 through 9 to the characters 0 through 9
map the numbers 10-99 to the characters a through i by dividing these values by 10
fragments in the range of 100-999 will be divided by 100 and mapped to the letters j through r
this only leaves us with the 8 letters s though z and being the coarsest will be mapped to powers of 10 by essentially dividing the value by 1000 and taking the log10() of it

This results in a display like the following:

collectl -sb -oT
#         <---Memory-->
#Time        Fragments
16:24:13   qmji9576040
16:24:14   rmji9576040
16:24:15   smji9576040
16:24:16   smji9576040
16:24:17   rmji9576040

As a final example of what this looks like when combined with other data, remembering you can chose virtually any combination of subsystem for your display both in collection and playback modes:

collectl -sbcmn -oT
#         <--------CPU--------><------------------Memory-----------------><----------Network---------->
#Time     cpu sys inter  ctxsw Free Buff Cach Inac Slab  Map    Fragments   KBIn  PktIn  KBOut  PktOut
16:44:46    0   0  1029    146  23M 178M   6G   5G 461M 234M  lljj9576040      2      8      0       2
16:44:47    0   0  1020    136  24M 178M   6G   5G 461M 234M  nljj9576040      2      8      1       2
16:44:48    1   0  1062    371  22M 178M   6G   5G 461M 235M  kljj9576040      3     31      2      27
16:44:49    0   0  1009    146  22M 178M   6G   5G 461M 235M  kljj9576040     14     13      0       2