Adjust arc_p based on "bytes" in arc_shrink

In an attempt to prevent arc_c from collapsing "too fast", the
arc_shrink() function was updated to take a "bytes" parameter by this
change:

    commit 302f753f1657c05a4287226eeda1f53ae431b8a7
    Author: Brian Behlendorf <behlendorf1@llnl.gov>
    Date:   Tue Mar 13 14:29:16 2012 -0700

        Integrate ARC more tightly with Linux

Unfortunately, that change failed to make a similar change to the way
that arc_p was updated. So, there still exists the possibility for arc_p
to collapse to near 0 when the kernel start calling the arc's shrinkers.

This change attempts to fix this, by decrementing arc_p by the "bytes"
parameter in the same way that arc_c is updated.

In addition, a minimum value of arc_p is attempted to be maintained,
similar to the way a minimum arc_p value is maintained in arc_adapt().

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2110

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 11839af59c9f607d4ae4b5ffdb560a76fabd5b53..cbc9b9feeb9738910d2b919fa3cf59230b290dce 100644 (file)
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -2332,6 +2332,7 @@ void
 arc_shrink(uint64_t bytes)
 {
        if (arc_c > arc_c_min) {
+               uint64_t arc_p_min;
                uint64_t to_free;
 
                to_free = bytes ? bytes : arc_c >> zfs_arc_shrink_shift;
@@ -2341,7 +2342,14 @@ arc_shrink(uint64_t bytes)
                else
                        arc_c = arc_c_min;
 
-               atomic_add_64(&arc_p, -(arc_p >> zfs_arc_shrink_shift));
+               arc_p_min = (arc_c >> zfs_arc_p_min_shift);
+               to_free = bytes ? bytes : arc_p >> zfs_arc_shrink_shift;
+
+               if (arc_p > arc_p_min + to_free)
+                       atomic_add_64(&arc_p, -to_free);
+               else
+                       arc_p = arc_p_min;
+
                if (arc_c > arc_size)
                        arc_c = MAX(arc_size, arc_c_min);
                if (arc_p > arc_c)