[dm-devel] [PATCH v5 1/1] multipath-tools: Prioritizer based on a latency algorithm
Yang Feng
philip.yang at huawei.com
Fri Jun 16 09:12:31 UTC 2017
Hi Martin,
Thanks for your reviews.
Please find my replys.
And the patch v6 will be sent later.
Regards,
Yang
---
>
>> +#define MAX_LATENCY_INTERVAL 10 /*unit: s*/
>> +#define MIN_LATENCY_INTERVAL 1 /*unit: us, or ms, or s*/
>
> Why not use the same unit or both values?
>
It's easy to check the valid range of the "latency_interval" value.
For unit: us, or ms, or s, the min digital value is the same, is 1.
And for all unit, the max conversion value is the same, is 10 seconds.
>> +static inline long long timeval_to_us(const struct timespec *tv)
>> +{
>> + return ((long long) tv->tv_sec * USEC_PER_SEC) + (tv-
>>> tv_nsec >> 10);
>
> Please calculate correctly, divide by 1000.
>
Thanks, it will be fixed.
>> +int check_args_valid(int io_num, long long latency_interval, int
>> type)
>> +{
>> + if ((io_num < MIN_IO_NUM) || (io_num > MAX_IO_NUM))
>> + {
>> + condlog(0, "args io_num is more than the valid values
>> range");
>> + return 0;
>> + }
>
> Please write "outside the valid range" rather than "more than the valid
> values range".
>
Thanks, it will be fixed.
> In general, when you use condlog(), please prefix all messages with a
> string that clearly identifies the path_latency prioritizer, e.g.
> PRIO_PATH_LATENCY.
> I know we're not currently doing that consistently in multipath-tools,
> but that shouldn't be a reason for not doing when adding new code.
>
Good, thanks, it will be fixed.
>> +
>> + /* check value is set by using logarithmic scale, and base
>> number is 10 */
>> + if ((latency_interval % BASE_NUMBER) != 0)
>> + {
>> + condlog(0, "args latency_interval is not set by using
>> logarithmic scale , where base number is 10");
>> + return 0;
>> + }
>
> This is not what I meant with "logarithmic scale". See below.
>
>> +
>> + /* s:[1, 10], ms:[1, 10000], us:[1, 10000000] */
>> + if ((latency_interval < MIN_LATENCY_INTERVAL)
>> + || (latency_interval > (MAX_LATENCY_INTERVAL * USEC_PER_SEC
>> / conversion_ratio[type])))
>> + {
>> + condlog(0, "args latency_interval is more than the valid
>> values range");
>> + return 0;
>> + }
>
> Again, please use "outside".
>
Thanks, it will be fixed.
>> +/* In multipath.conf, args form: io_num|latency_interval. For
>> example,
>> +* args is "20|10ms", this function can get 20, 10.
>> +*/
>> +static int get_interval_and_ionum(char *args,
>> + int *ionum,
>> + long long *interval)
>> +{
>> + char source[MAX_CHAR_SIZE];
>> + char vertica = '|';
>> + char *endstrbefore = NULL;
>> + char *endstrafter = NULL;
>> + int type;
>> + unsigned int size = strlen(args);
>> + long long ratio;
>> +
>> + if ((args == NULL) || (ionum == NULL) || (interval == NULL))
>> + {
>> + condlog(0, "args string is NULL");
>> + return 0;
>> + }
>> +
>> + if ((size < 1) || (size > MAX_CHAR_SIZE-1))
>> + {
>> + condlog(0, "args string's size is too long");
>> + return 0;
>> + }
>
> "too long" is not correct for the first (more likely) error condition.
>
>> +
>> + memcpy(source, args, size+1);
>> +
>> + if (!isdigit(source[0]))
>> + {
>> + condlog(0, "args io_num string's first char is not digit");
>> + return 0;
>> + }
>
> It would be ok to catch all these errors with a single line of code
> such as "path_latency: invalid prio_args format: %s".
>
OK, it will be fixed.
>> +
>> + *ionum = (int)strtoul(source, &endstrbefore, 10);
>> + if (endstrbefore[0] != vertica)
>> + {
>> + condlog(0, "segmentation char is invalid");
>> + return 0;
>> + }
>> +
>> + if (!isdigit(endstrbefore[1]))
>> + {
>> + condlog(0, "args latency_interval string's first char is not
>> digit");
>> + return 0;
>> + }
>> +
>> + *interval = (long long)strtol(&endstrbefore[1], &endstrafter,
>> 10);
>> + type = get_interval_type(endstrafter);
>> + if (type == INTERVAL_INVALID)
>> + {
>> + condlog(0, "args latency_interval type is invalid");
>> + return 0;
>> + }
>> +
>> + if (check_args_valid(*ionum, *interval, type) == 0)
>> + {
>> + return 0;
>> + }
>> +
>> + ratio = get_conversion_ratio(type);
>> + *interval *= (long long)ratio;
>> +
>> + return 1;
>> +}
>> +
>> +long long calc_standard_deviation(long long *path_latency, int size,
>> long long avglatency)
>> +{
>> + int index;
>> + long long total = 0;
>> +
>> + for (index = 0; index < size; index++)
>> + {
>> + total += (path_latency[index] - avglatency) *
>> (path_latency[index] - avglatency);
>> + }
>> +
>> + total /= (size-1);
>> +
>> + return (long long)sqrt((double)total);
>> +}
>> +
>> +int getprio (struct path *pp, char *args, unsigned int timeout)
>> +{
>> + int rc, temp;
>> + int index = 0;
>> + int io_num;
>> + long long latency_interval;
>> + long long avglatency;
>> + long long standard_deviation;
>> + long long toldelay = 0;
>> + long long before, after;
>> + struct timespec tv;
>> +
>> + if (pp->fd < 0)
>> + return -1;
>> +
>> + if (get_interval_and_ionum(args, &io_num, &latency_interval) ==
>> 0)
>> + {
>> + condlog(0, "%s: get path_latency args fail", pp->dev);
>> + return DEFAULT_PRIORITY;
>> + }
>
> Could you just assume reasonable defaults for the path latency
> algorithm rather than returning a constant?
>
En, I think returning a DEFAULT_PRIORITY "0" is no problem.
If return a assumed value, then the user can be confused, because
the defaults is not set.
>> +
>> + memset(path_latency, 0, sizeof(path_latency));
>> +
>> + temp = io_num;
>> + while (temp-- > 0)
>> + {
>> + (void)clock_gettime(CLOCK_MONOTONIC, &tv);
>> + before = timeval_to_us(&tv);
>> +
>> + if (do_readsector0(pp->fd, timeout) == 2)
>> + {
>> + condlog(0, "%s: path down", pp->dev);
>> + return -1;
>> + }
>> +
>> + (void)clock_gettime(CLOCK_MONOTONIC, &tv);
>> + after = timeval_to_us(&tv);
>> +
>> + path_latency[index] = after - before;
>> + toldelay += path_latency[index++];
>> + }
>> +
>> + avglatency = toldelay/(long long)io_num;
>> + condlog(4, "%s: average latency is (%lld)", pp->dev,
>> avglatency);
>> +
>> + if (avglatency > THRES_USEC_VALUE)
>> + {
>> + condlog(0, "%s: average latency (%lld) is more than
>> thresold", pp->dev, avglatency);
>> + return DEFAULT_PRIORITY;
>> + }
>
> Why don't you put simply put this in the highest latency bin? See
> below.
>
Thanks, it will be fixed.
>> +
>> + /* warn the user if the latency_interval set is smaller than (2
>> * standard deviation), or equal */
>> + standard_deviation = calc_standard_deviation(path_latency,
>> index, avglatency);
>> + if (latency_interval <= (2 * standard_deviation))
>> + condlog(3, "%s: args latency_interval set is smaller than 2
>> * standard deviation (%lld us), or equal",
>> + pp->dev, standard_deviation);
>> +
>> + rc = (int)(THRES_USEC_VALUE - (avglatency/(long
>> long)latency_interval));
>> + return rc;
>> +}
>
> Hm, I think you misunderstood my "logarithmic scale" idea.
> By dividing the average by the interval size, you get ordinary linear
> scaling again. That doesn't help the problem.
>
> Here is a sample program illustrating what I meant. It uses math.h
> functions, but that shouldn't be a problem; we're running in user
> space. The chosen interval size, corresponding to a factor of 1.995, is
> probably a good choice for many scenarios. Your could represent the
> range between 1us and 10s with 34 prio values, and still be able to
> differentiate paths with, say, 1.5 and 3 us latency.
>
> If you use this, you should also use double type for calculating the
> average and stddev, and you must be careful to calculate the standard
> deviation for the logarithms, as interval size is not constant any more
> (only on logarithmic scale).
>
> #include <stdio.h>
> #include <math.h>
>
> /*
> * For a given positive value x, return a "prio" between 0 and n
> * using logarithmically scaled intervals between minval and maxval.
> */
>
> int log_prio(double x,
> unsigned long minval, unsigned long maxval, unsigned n)
> {
> double lx, lmin, lmax;
> if (x <= minval)
> return n;
> if (x >= maxval)
> return 0;
> lx = log10(x);
> lmin = log10(minval);
> lmax = log10(maxval);
> return n - (n - 1) * (lx - lmin) / (lmax - lmin);
> }
>
> int main(void)
> {
> int minval = 1;
> int maxval = 1000;
> int n = 11;
> double x;
> for (x = minval/2.; x < maxval*2.; x *= sqrt(2.))
> printf("%f: %d\n", x, log_prio(x, minval, maxval, n));
> return 0;
> }
>
OK, the argument "latency_interval" will be delete, instead, the argument
"base_num" will be added for logarithmic scale.
And min value will be set 1 us, max value will be set 10000000 us (or 10 s).
> Regards,
> Martin
>
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