Hi again Reinhard, On Fri, Nov 25, 2016 at 09:17:41AM +0100, Reinhard Vicinus wrote: > Hi Willy, > > if the cost are to high, then I have no problem keeping the known > behavior. The only thing I would suggest is to document it, because it > caused me some headache to figure out why the values were always to low > and I couldn't find any information, that this behavior is a know problem.
I finally found a much more elegant solution by improving the formula to save one multiply. Not only does this avoid the overflow without changing the integer size, it's also faster :-) Now the limit is at 8.4M milliseconds of average time, or around 2h18m, this should be plenty for most situations! I'm attaching the patch I've just merged for this. Best regards, Willy
>From 3758581e197dd7b390fb3da94c08f34e0d319c07 Mon Sep 17 00:00:00 2001 From: Willy Tarreau <[email protected]> Date: Fri, 25 Nov 2016 11:55:10 +0100 Subject: BUG/MINOR: freq-ctr: make swrate_add() support larger values X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Reinhard Vicinus reported that the reported average response times cannot be larger than 16s due to the double multiply being performed by swrate_add() which causes an overflow very quickly. Indeed, with N=512, the highest average value is 16448. One solution proposed by Reinhard is to turn to long long, but this involves 64x64 multiplies and 64->32 divides, which are extremely expensive on 32-bit platforms. There is in fact another way to avoid the overflow without using larger integers, it consists in avoiding the multiply using the fact that x*(n-1)/N = x-(x/N). Now it becomes possible to store average values as large as 8.4 millions, which is around 2h18mn. Interestingly, this improvement also makes the code cheaper to execute both on 32 and on 64 bit platforms : Before : 00000000 <swrate_add>: 0: 8b 54 24 04 mov 0x4(%esp),%edx 4: 8b 0a mov (%edx),%ecx 6: 89 c8 mov %ecx,%eax 8: c1 e0 09 shl $0x9,%eax b: 29 c8 sub %ecx,%eax d: 8b 4c 24 0c mov 0xc(%esp),%ecx 11: c1 e8 09 shr $0x9,%eax 14: 01 c8 add %ecx,%eax 16: 89 02 mov %eax,(%edx) After : 00000020 <swrate_add>: 20: 8b 4c 24 04 mov 0x4(%esp),%ecx 24: 8b 44 24 0c mov 0xc(%esp),%eax 28: 8b 11 mov (%ecx),%edx 2a: 01 d0 add %edx,%eax 2c: 81 c2 ff 01 00 00 add $0x1ff,%edx 32: c1 ea 09 shr $0x9,%edx 35: 29 d0 sub %edx,%eax 37: 89 01 mov %eax,(%ecx) This fix may be backported to 1.6. --- include/proto/freq_ctr.h | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/include/proto/freq_ctr.h b/include/proto/freq_ctr.h index 65388b1..70b295e 100644 --- a/include/proto/freq_ctr.h +++ b/include/proto/freq_ctr.h @@ -182,7 +182,19 @@ unsigned int freq_ctr_remain_period(struct freq_ctr_period *ctr, unsigned int pe * * So basically by summing values and applying the last result an (N-1)/N factor * we just get N times the values over the long term, so we can recover the - * constant value V by dividing by N. + * constant value V by dividing by N. In order to limit the impact of integer + * overflows, we'll use this equivalence which saves us one multiply : + * + * N - 1 1 x0 + * x1 = x0 * ------- = x0 * ( 1 - --- ) = x0 - ---- + * N N N + * + * And given that x0 is discrete here we'll have to saturate the values before + * performing the divide, so the value insertion will become : + * + * x0 + N - 1 + * x1 = x0 - ------------ + * N * * A value added at the entry of the sliding window of N values will thus be * reduced to 1/e or 36.7% after N terms have been added. After a second batch, @@ -220,7 +232,7 @@ unsigned int freq_ctr_remain_period(struct freq_ctr_period *ctr, unsigned int pe */ static inline unsigned int swrate_add(unsigned int *sum, unsigned int n, unsigned int v) { - return *sum = *sum * (n - 1) / n + v; + return *sum = *sum - (*sum + n - 1) / n + v; } /* Returns the average sample value for the sum <sum> over a sliding window of -- 1.7.12.1
