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Commit fe5cbc6e authored by Markus Stockhausen's avatar Markus Stockhausen Committed by NeilBrown
Browse files

md/raid6 algorithms: delta syndrome functions 



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: default avatarMarkus Stockhausen <stockhausen@collogia.de>
Signed-off-by: default avatarNeilBrown <neilb@suse.de>
parent dabc4ec6

include/linux/raid/pq.h

+1 −0
Original line number Diff line number Diff line
@@ -72,6 +72,7 @@ extern const char raid6_empty_zero_page[PAGE_SIZE]; 
/* Routine choices */

struct raid6_calls {

	void (*gen_syndrome)(int, size_t, void **);

	void (*xor_syndrome)(int, int, int, size_t, void **);

	int  (*valid)(void);	/* Returns 1 if this routine set is usable */

	const char *name;	/* Name of this routine set */

	int prefer;		/* Has special performance attribute */

lib/raid6/algos.c

+34 −7
Original line number Diff line number Diff line
@@ -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");

lib/raid6/altivec.uc

+1 −0
Original line number Diff line number Diff line
@@ -119,6 +119,7 @@ int raid6_have_altivec(void) 


const struct raid6_calls raid6_altivec$# = {

	raid6_altivec$#_gen_syndrome,

	NULL,			/* XOR not yet implemented */

	raid6_have_altivec,

	"altivecx$#",

	0

lib/raid6/avx2.c

+3 −0
Original line number Diff line number Diff line
@@ -89,6 +89,7 @@ static void raid6_avx21_gen_syndrome(int disks, size_t bytes, void **ptrs) 


const struct raid6_calls raid6_avx2x1 = {

	raid6_avx21_gen_syndrome,

	NULL,			/* XOR not yet implemented */

	raid6_have_avx2,

	"avx2x1",

	1			/* Has cache hints */
@@ -150,6 +151,7 @@ static void raid6_avx22_gen_syndrome(int disks, size_t bytes, void **ptrs) 


const struct raid6_calls raid6_avx2x2 = {

	raid6_avx22_gen_syndrome,

	NULL,			/* XOR not yet implemented */

	raid6_have_avx2,

	"avx2x2",

	1			/* Has cache hints */
@@ -242,6 +244,7 @@ static void raid6_avx24_gen_syndrome(int disks, size_t bytes, void **ptrs) 


const struct raid6_calls raid6_avx2x4 = {

	raid6_avx24_gen_syndrome,

	NULL,			/* XOR not yet implemented */

	raid6_have_avx2,

	"avx2x4",

	1			/* Has cache hints */

lib/raid6/int.uc

+2 −1
Original line number Diff line number Diff line
@@ -109,6 +109,7 @@ static void raid6_int$#_gen_syndrome(int disks, size_t bytes, void **ptrs) 


const struct raid6_calls raid6_intx$# = {

	raid6_int$#_gen_syndrome,

	NULL,			/* XOR not yet implemented */

	NULL,			/* always valid */

	"int" NSTRING "x$#",

	0