I have been trying to understand a behavior I am observing in an IOR
benchmark on Lustre. I have pared it down to a simple example.
The IOR benchmark is running in MPI mode. There are 2 ranks, each
running on its own node. Each rank does the following:
Note : Test was run on the "swan" cluster at Cray Inc., using /lus/scratch
write a file. ( 10GB )
fsync the file
close the file
open the file that was written by the other rank.
read the file that was written by the other rank.
close the file that was written by the other rank.
The writing of each file goes as expected.
The fsync takes very little time ( about .05 seconds).
The first reads of the file( written by the other rank ) start out *very
*slowly. While theses first reads are proceeding slowly, the
kernel's cached memory ( the Cached: line in /proc/meminfo) decreases
from the size of the file just written to nearly zero.
Once the cached memory has reached nearly zero, the file reading
proceeds as expected.
I have attached a jpg of the instrumentation of the processes that
illustrates this behavior.
My questions are:
Why does the reading of the file, written by the other rank, wait until
the cached data drains to nearly zero before proceeding normally.
Shouldn't the fsync ensure that the file's data is written to the
backing storage so this draining of the cached memory should be simply
releasing pages with no further I/O?
For this case the "dead" time is only about 4 seconds, but this "dead"
time scales directly with the size of the files.
I/O Doctors LLC