Assessing paging space I/O with the vmstat command

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The vmstat reports indicate the amount of paging-space I/O taking place.

I/O to and from paging spaces is random, mostly one page at a time. Both of the following examples show the paging activity that occurs during a C compilation in a machine that has been artificially shrunk using the rmss command. The pi and po (paging-space page-ins and paging-space page-outs) columns show the amount of paging-space I/O (in terms of 4096-byte pages) during each 5-second interval. The first report (summary since system reboot) has been removed. Notice that the paging activity occurs in bursts. 
# vmstat 5 8
kthr     memory             page              faults        cpu
----- ----------- ------------------------ ------------ -----------
 r  b   avm   fre  re  pi  po  fr   sr  cy  in   sy  cs us sy id wa
 0  1 72379   434   0   0   0   0    2   0 376  192 478  9  3 87  1
 0  1 72379   391   0   8   0   0    0   0 631 2967 775 10  1 83  6
 0  1 72379   391   0   0   0   0    0   0 625 2672 790  5  3 92  0
 0  1 72379   175   0   7   0   0    0   0 721 3215 868  8  4 72 16
 2  1 71384   877   0  12  13  44  150   0 662 3049 853  7 12 40 41
 0  2 71929   127   0  35  30 182  666   0 709 2838 977 15 13  0 71
 0  1 71938   122   0   0   8  32  122   0 608 3332 787 10  4 75 11
 0  1 71938   122   0   0   0   3   12   0 611 2834 733  5  3 75 17

The following "before and after" vmstat -s reports show the accumulation of paging activity. Remember that it is the paging space page ins and paging space page outs that represent true paging-space I/O. The (unqualified) page ins and page outs report total I/O, that is both paging-space I/O and the ordinary file I/O, performed by the paging mechanism. The reports have been edited to remove lines that are irrelevant to this discussion.

# vmstat -s # before # vmstat -s # after
6602 page ins 3948 page outs 544 paging space page ins 1923 paging space page outs 0 total reclaims 7022 page ins 4146 page outs 689 paging space page ins 2032 paging space page outs 0 total reclaims

The fact that more paging-space page-ins than page-outs occurred during the compilation suggests that we had shrunk the system to the point that thrashing begins. Some pages were being repaged because their frames were stolen before their use was complete.