From fe5cbc6e06c7d8b3a86f6f5491d74766bb5c2827 Mon Sep 17 00:00:00 2001 From: Markus Stockhausen Date: Mon, 15 Dec 2014 12:57:04 +1100 Subject: 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: Markus Stockhausen Signed-off-by: NeilBrown --- lib/raid6/algos.c | 41 ++++++++++++++++++++++++++++++++++------- 1 file changed, 34 insertions(+), 7 deletions(-) (limited to 'lib/raid6/algos.c') diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c index dbef2314901e..975c6e0434bd 100644 --- a/lib/raid6/algos.c +++ b/lib/raid6/algos.c @@ -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<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"); -- cgit v1.2.3-70-g09d2