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md/raid6 algorithms: delta syndrome functions

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v3: s-o-b comment, explanation of performance and descision for
the start/stop implementation

Implementing rmw functionality for RAID6 requires optimized syndrome
calculation. Up to now we can only generate a complete syndrome. The
target P/Q pages are always overwritten. With this patch we provide
a framework for inplace P/Q modification. In the first place simply
fill those functions with NULL values.

xor_syndrome() has two additional parameters: start & stop. These
will indicate the first and last page that are changing during a
rmw run. That makes it possible to avoid several unneccessary loops
and speed up calculation. The caller needs to implement the following
logic to make the functions work.

1) xor_syndrome(disks, start, stop, ...): "Remove" all data of source
blocks inside P/Q between (and including) start and end.

2) modify any block with start <= block <= stop

3) xor_syndrome(disks, start, stop, ...): "Reinsert" all data of
source blocks into P/Q between (and including) start and end.

Pages between start and stop that won't be changed should be filled
with a pointer to the kernel zero page. The reasons for not taking NULL
pages are:

1) Algorithms cross the whole source data line by line. Thus avoid
additional branches.

2) Having a NULL page avoids calculating the XOR P parity but still
need calulation steps for the Q parity. Depending on the algorithm
unrolling that might be only a difference of 2 instructions per loop.

The benchmark numbers of the gen_syndrome() functions are displayed in
the kernel log. Do the same for the xor_syndrome() functions. This
will help to analyze performance problems and give an rough estimate
how well the algorithm works. The choice of the fastest algorithm will
still depend on the gen_syndrome() performance.

With the start/stop page implementation the speed can vary a lot in real
life. E.g. a change of page 0 & page 15 on a stripe will be harder to
compute than the case where page 0 & page 1 are XOR candidates. To be not
to enthusiatic about the expected speeds we will run a worse case test
that simulates a change on the upper half of the stripe. So we do:

1) calculation of P/Q for the upper pages

2) continuation of Q for the lower (empty) pages

Signed-off-by: Markus Stockhausen <stockhausen@collogia.de>
Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
Markus Stockhausen 2014-12-15 12:57:04 +11:00 • committed by NeilBrown
parent dabc4ec6ba
commit fe5cbc6e06
10 changed files with 50 additions and 8 deletions
Download patch file Download diff file Expand all files Collapse all files

41
lib/raid6/algos.c
View file

@ -131,11 +131,12 @@ static inline const struct raid6_recov_calls *raid6_choose_recov(void)
static inline const struct raid6_calls *raid6_choose_gen(
void *(*const dptrs)[(65536/PAGE_SIZE)+2], const int disks)
{
unsigned long perf, bestperf, j0, j1;
unsigned long perf, bestgenperf, bestxorperf, j0, j1;
int start = (disks>>1)-1, stop = disks-3; /* work on the second half of the disks */
const struct raid6_calls *const *algo;
const struct raid6_calls *best;
for (bestperf = 0, best = NULL, algo = raid6_algos; *algo; algo++) {
for (bestgenperf = 0, bestxorperf = 0, best = NULL, algo = raid6_algos; *algo; algo++) {
if (!best || (*algo)->prefer >= best->prefer) {
if ((*algo)->valid && !(*algo)->valid())
continue;
@ -153,19 +154,45 @@ static inline const struct raid6_calls *raid6_choose_gen(
}
preempt_enable();
if (perf > bestperf) {
bestperf = perf;
if (perf > bestgenperf) {
bestgenperf = perf;
best = *algo;
}
pr_info("raid6: %-8s %5ld MB/s\n", (*algo)->name,
pr_info("raid6: %-8s gen() %5ld MB/s\n", (*algo)->name,
(perf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2));
if (!(*algo)->xor_syndrome)
continue;
perf = 0;
preempt_disable();
j0 = jiffies;
while ((j1 = jiffies) == j0)
cpu_relax();
while (time_before(jiffies,
j1 + (1<<RAID6_TIME_JIFFIES_LG2))) {
(*algo)->xor_syndrome(disks, start, stop,
PAGE_SIZE, *dptrs);
perf++;
}
preempt_enable();
if (best == *algo)
bestxorperf = perf;
pr_info("raid6: %-8s xor() %5ld MB/s\n", (*algo)->name,
(perf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2+1));
}
}
if (best) {
pr_info("raid6: using algorithm %s (%ld MB/s)\n",
pr_info("raid6: using algorithm %s gen() %ld MB/s\n",
best->name,
(bestperf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2));
(bestgenperf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2));
if (best->xor_syndrome)
pr_info("raid6: .... xor() %ld MB/s, rmw enabled\n",
(bestxorperf*HZ) >> (20-16+RAID6_TIME_JIFFIES_LG2+1));
raid6_call = *best;
} else
pr_err("raid6: Yikes! No algorithm found!\n");