The memory permutation cause a slow down in case of a save/restore (bug
1143). It works better when the mixing is done with 2MB chunks.
Signed-off-by: Jean Guyader <jean.guyader@xxxxxxxxxxxxx>
diff -r 59b8768d0d0d tools/libxc/xc_domain_save.c
--- a/tools/libxc/xc_domain_save.c Wed Mar 05 11:18:25 2008 +0000
+++ b/tools/libxc/xc_domain_save.c Tue Mar 18 17:45:03 2008 +0000
@@ -125,34 +125,22 @@ static inline int count_bits ( int nr, v
return count;
}
-static inline int permute( int i, int nr, int order_nr )
+static inline int permute(unsigned long i, unsigned long order_nr)
{
/* Need a simple permutation function so that we scan pages in a
pseudo random order, enabling us to get a better estimate of
the domain's page dirtying rate as we go (there are often
contiguous ranges of pfns that have similar behaviour, and we
want to mix them up. */
+
+ unsigned char keep = 9; /* chunk of 2 MB */
+ unsigned char shift_low = (order_nr - keep) / 2 + ((order_nr - keep) / 2) %
2;
+ unsigned char shift_high = order_nr - keep - shift_low;
- /* e.g. nr->oder 15->4 16->4 17->5 */
- /* 512MB domain, 128k pages, order 17 */
+ unsigned long high = (i >> (keep + shift_low));
+ unsigned long low = (i >> keep) & ((1 << shift_low) - 1);
- /*
- QPONMLKJIHGFEDCBA
- QPONMLKJIH
- GFEDCBA
- */
-
- /*
- QPONMLKJIHGFEDCBA
- EDCBA
- QPONM
- LKJIHGF
- */
-
- do { i = ((i>>(order_nr-10)) | ( i<<10 ) ) & ((1<<order_nr)-1); }
- while ( i >= nr ); /* this won't ever loop if nr is a power of 2 */
-
- return i;
+ return (i & ((1 << keep) - 1)) | (low << (shift_high + keep)) | (high <<
keep);
}
static uint64_t tv_to_us(struct timeval *new)
@@ -1126,7 +1114,7 @@ int xc_domain_save(int xc_handle, int io
(batch < MAX_BATCH_SIZE) && (N < p2m_size);
N++ )
{
- int n = permute(N, p2m_size, order_nr);
+ int n = permute(N, order_nr);
if ( debug )
{
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