RE: frontswap/zcache: xvmalloc discussion

FWIW, I've measured the distribution of zsize (pages compressed
with frontswap) on my favorite workload (kernel "make -j2" on
mem=512M to force lots of swapping) and the mean is small, close
to 1K (PAGE_SIZE/4).  I've added some sysfs shows for both
the current and cumulative distribution (0-63 bytes, 64-127
bytes, ..., 4032-4095 bytes) for the next update.

I tried your program on the text of Moby Dick and the mean
was still under 1500 bytes ((3*PAGE_SIZE)/8) with a good
broad distribution for zsize.

Oops, on retry this morning, I am now clearly seeing the poor
compression.  Not sure what is different from last night,
but I suspect I was "watch"ing the new sysfs output during
massive swapping during the run of the test program and
it wasn't updated until program completion (at which point
I was no longer perusing the sysfs output).

Sorry for the noise.  However, now that I have a test case
I am implementing another sysfs tunable to reject poorly-
compressible pages that would drive the mean zsize
above the tunable.  Zcache will reject these pages until
the mean falls below the threshold.  (Setting it to
PAGE_SIZE will continue current behavior, but I've set
the default to (5*PAGE_SIZE)/8 for now.)

Dan

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