On 3/31/23 06:39, Han Zhou wrote:
>
>
> On Wed, Mar 29, 2023 at 9:05 AM Ilya Maximets <[email protected]
> <mailto:[email protected]>> wrote:
>>
>> While crushing OR expressions, OVN removes exact replicas of sub
>> expressions. However, there could be many CMP expressions that are
>> supersets of each other. These are most likely to be created as a
>> result of cross-product while expanding brackets in the AND expression
>> in crush_and_numeric(), i.e. while converting
>> "x && (a0 || a1) && (b0 || b1)" into "xa0b0 || xa0b1 || xa1b0 || xa1b1".
>>
>> In addition to removal of exact duplicates introducing scan and removal
>> of supersets of other existing sub-expressions to reduce the amount of
>> generated flows. This adds extra computations, but should save time
>> later, since less flows will be generated.
>>
>> Example:
>>
>> "ip4.src == 172.168.0.0/16 <http://172.168.0.0/16> &&
>> ip4.src!={172.168.13.0/24 <http://172.168.13.0/24>, 172.168.15.0/24
>> <http://172.168.15.0/24>}"
>>
>> Processing of this expression yields 42 flows:
>>
>> $ ./tests/ovstest test-ovn expr-to-flows <<< "$expr"
>>
>> ip,nw_src=172.168.0.0/255.255.1.0 <http://172.168.0.0/255.255.1.0>
>> ip,nw_src=172.168.0.0/255.255.10.0 <http://172.168.0.0/255.255.10.0>
>> ip,nw_src=172.168.0.0/255.255.12.0 <http://172.168.0.0/255.255.12.0>
>> ip,nw_src=172.168.0.0/255.255.3.0 <http://172.168.0.0/255.255.3.0>
>> ip,nw_src=172.168.0.0/255.255.4.0 <http://172.168.0.0/255.255.4.0>
>> ip,nw_src=172.168.0.0/255.255.5.0 <http://172.168.0.0/255.255.5.0>
>> ip,nw_src=172.168.0.0/255.255.6.0 <http://172.168.0.0/255.255.6.0>
>> ip,nw_src=172.168.0.0/255.255.8.0 <http://172.168.0.0/255.255.8.0>
>> ip,nw_src=172.168.0.0/255.255.9.0 <http://172.168.0.0/255.255.9.0>
>> ip,nw_src=172.168.128.0/17 <http://172.168.128.0/17>
>> <... 32 more flows ...>
>>
>> We can see that many flows above do overlap, e.g. 255.255.3.0
>> mask is a superset of 255.255.1.0. Everything that matches
>> 255.255.3.0, will match 255.255.1.0 as well (the value is the same).
>>
>> By removing all the unnecessary supersets, the set of flows can
>> be reduced from 42 down to 7:
>>
>> ip,nw_src=172.168.0.0/255.255.1.0 <http://172.168.0.0/255.255.1.0>
>> ip,nw_src=172.168.0.0/255.255.4.0 <http://172.168.0.0/255.255.4.0>
>> ip,nw_src=172.168.0.0/255.255.8.0 <http://172.168.0.0/255.255.8.0>
>> ip,nw_src=172.168.128.0/17 <http://172.168.128.0/17>
>> ip,nw_src=172.168.16.0/255.255.16.0 <http://172.168.16.0/255.255.16.0>
>> ip,nw_src=172.168.32.0/255.255.32.0 <http://172.168.32.0/255.255.32.0>
>> ip,nw_src=172.168.64.0/255.255.64.0 <http://172.168.64.0/255.255.64.0>
>>
>> This change should be particularly useful for expressions with
>> inequality checks, like the one above. Such expressions are
>> frequent among ACL rules.
>>
>> "ip4.src != {172.168.13.0/24 <http://172.168.13.0/24>, 172.168.14.0/24
>> <http://172.168.14.0/24>, 172.168.15.0/24 <http://172.168.15.0/24>}"
>>
>> Brefore:
>> $ ./tests/ovstest test-ovn expr-to-flows <<< "$expr" | wc -l
>> 2894
>>
>> After:
>> $ ./tests/ovstest test-ovn expr-to-flows <<< "$expr" | wc -l
>> 23
>>
>> Superset lookups are performed only if there are expressions with
>> more bits in the mask than in the current one. So, there is no extra
>> cost for equality checks on normal address sets, like port group sets,
>> where all the IPs are exect matches or have the same prefix length
>> otherwise.
>>
>> Use of bitmaps instead of subvalue functions significantly speeds up
>> processing since most of the subvalue space is an all-zero empty space.
>>
>> Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=2177197
>> <https://bugzilla.redhat.com/show_bug.cgi?id=2177197>
>> Reported-by: Nadia Pinaeva <[email protected] <mailto:[email protected]>>
>> Signed-off-by: Ilya Maximets <[email protected] <mailto:[email protected]>>
>> ---
>> include/ovn/expr.h | 1 +
>> lib/expr.c | 238 ++++++++++++++++++++++++++++++++++-----------
>> tests/ovn.at <http://ovn.at> | 12 +++
>> 3 files changed, 196 insertions(+), 55 deletions(-)
>>
>> diff --git a/include/ovn/expr.h b/include/ovn/expr.h
>> index 80d95ff67..c48f82398 100644
>> --- a/include/ovn/expr.h
>> +++ b/include/ovn/expr.h
>> @@ -384,6 +384,7 @@ struct expr {
>> struct {
>> union mf_subvalue value;
>> union mf_subvalue mask;
>> + size_t mask_n_bits;
>> };
>> };
>> } cmp;
>> diff --git a/lib/expr.c b/lib/expr.c
>> index 0a6f7e574..8fd07b2d5 100644
>> --- a/lib/expr.c
>> +++ b/lib/expr.c
>> @@ -15,21 +15,23 @@
>> */
>>
>> #include <config.h>
>> +#include "bitmap.h"
>> #include "byte-order.h"
>> -#include "openvswitch/json.h"
>> +#include "hmapx.h"
>> #include "nx-match.h"
>> #include "openvswitch/dynamic-string.h"
>> +#include "openvswitch/json.h"
>> #include "openvswitch/match.h"
>> #include "openvswitch/ofp-actions.h"
>> -#include "openvswitch/vlog.h"
>> #include "openvswitch/shash.h"
>> +#include "openvswitch/vlog.h"
>> +#include "ovn-util.h"
>> #include "ovn/expr.h"
>> #include "ovn/lex.h"
>> #include "ovn/logical-fields.h"
>> #include "simap.h"
>> #include "sset.h"
>> #include "util.h"
>> -#include "ovn-util.h"
>>
>> VLOG_DEFINE_THIS_MODULE(expr);
>>
>> @@ -2520,6 +2522,183 @@ crush_and_string(struct expr *expr, const struct
>> expr_symbol *symbol)
>> return expr_fix(expr);
>> }
>>
>> +static int
>> +compare_cmps_3way(const struct expr *a, const struct expr *b)
>> +{
>> + ovs_assert(a->cmp.symbol == b->cmp.symbol);
>> + if (!a->cmp.symbol->width) {
>> + return strcmp(a->cmp.string, b->cmp.string);
>> + } else if (a->cmp.mask_n_bits != b->cmp.mask_n_bits) {
>> + return a->cmp.mask_n_bits < b->cmp.mask_n_bits ? -1 : 1;
>> + } else {
>> + int d = memcmp(&a->cmp.value, &b->cmp.value, sizeof a->cmp.value);
>> + if (!d) {
>> + d = memcmp(&a->cmp.mask, &b->cmp.mask, sizeof a->cmp.mask);
>> + }
>> + return d;
>> + }
>> +}
>> +
>> +static int
>> +compare_cmps_cb(const void *a_, const void *b_)
>> +{
>> + const struct expr *const *ap = a_;
>> + const struct expr *const *bp = b_;
>> + const struct expr *a = *ap;
>> + const struct expr *b = *bp;
>> + return compare_cmps_3way(a, b);
>> +}
>> +
>> +/* Similar to mf_subvalue_intersect(), but only checks the possibility of
>> + * intersection without producing a result. */
>> +static bool
>> +expr_bitmap_intersect_check(const unsigned long *a_value,
>> + const unsigned long *a_mask,
>> + const unsigned long *b_value,
>> + const unsigned long *b_mask,
>> + size_t bit_width)
>> +{
>> + for (size_t i = 0; i < bitmap_n_longs(bit_width); i++) {
>> + if ((a_value[i] ^ b_value[i]) & (a_mask[i] & b_mask[i])) {
>> + return false;
>> + }
>> + }
>> + return true;
>> +}
>> +
>> +/* This function expects an OR expression with already crushed sub
>> + * expressions, so they are plain comparisons. Result is the same
>> + * expression, but with unnecessary sub-expressions removed. */
>> +static struct expr *
>> +crush_or_supersets(struct expr *expr, const struct expr_symbol *symbol)
>> +{
>> + ovs_assert(expr->type == EXPR_T_OR);
>> +
>> + /* Calculate offset within subfield and a width that can be used
>> + * in a bitmap. */
>> + const size_t sz = CHAR_BIT * sizeof expr->cmp.value.be64[0];
>> + const size_t bit_width = ROUND_UP(symbol->width, sz);
>> + const size_t ofs = ARRAY_SIZE(expr->cmp.value.be64) - bit_width / sz;
>> +
>> + /* Sort subexpressions by number of bits in the mask, value and the mask
>> + * itself, to bring together duplicates and have expressions ordered by
>> + * mask sizes. */
>> + size_t n = ovs_list_size(&expr->andor);
>> + struct expr **subs = xmalloc(n * sizeof *subs);
>> + size_t *next = xmalloc(n * sizeof *next);
>
> I spent some time figuring out that the |next| array is used to keep track of
> the next valid index within the |subs| array after processing the current
> index. It helps in optimizing the traversal of the |subs| array by skipping
> over the invalidated (or removed) sub-expressions during the elimination of
> duplicates and supersets. I think it would be helpful to have a comment for
> the readers.
In general, it's a way of implementing array-based linked list.
I can add a comment, sure.
>
>> +
>> + size_t i = 0, j, max_n_bits = 0;
>> + struct expr *sub;
>> + LIST_FOR_EACH (sub, node, &expr->andor) {
>> + if (symbol->width) {
>> + const unsigned long *sub_mask;
>> +
>> + sub_mask = (unsigned long *) &sub->cmp.mask.be64[ofs];
>> + sub->cmp.mask_n_bits = bitmap_count1(sub_mask, bit_width);
>> + max_n_bits = MAX(max_n_bits, sub->cmp.mask_n_bits);
>> + }
>> + next[i] = i + 1;
>> + subs[i++] = sub;
>> + }
>> + ovs_assert(i == n);
>> +
>> + qsort(subs, n, sizeof *subs, compare_cmps_cb);
>> +
>> + /* Eliminate duplicates. */
>> + size_t last = 0;
>> + for (i = 1; i < n; i++) {
>> + if (compare_cmps_3way(subs[last], subs[i])) {
>> + next[last] = i;
>> + last = i;
>> + } else {
>> + expr_destroy(subs[i]);
>> + subs[i] = NULL;
>> + }
>> + }
>> +
>> + if (!symbol->width || symbol->level != EXPR_L_ORDINAL) {
>> + /* Not a fully maskable field. Can't do much more. */
>> + goto done;
>> + }
>> +
>> + /* Build a mask size index. 'mask_index[n_bits]' is an index in 'subs',
>> + * where expressions with 'n_bits' bits in mask start. */
>> + size_t *mask_index, n_bits;
>> + size_t index_size = (max_n_bits + 2) * sizeof *mask_index;
>> +
>> + mask_index = xmalloc(index_size);
>> + /* Initialize to maximum unsigned values. */
>> + memset(mask_index, 0xf, index_size);
>
> 0xf is a little weird here. It is not wrong, just as correct as using even
> 0x1, but I think using 0xff, the max value of a byte, would make it more
> straightforward to understand.
Yeah, I meant to use 0xff. Not sure why I ended up with 0xf.
Will fix.
>
>> +
>> + for (i = 0; i < n; i = next[i]) {
>> + if (subs[i]) {
>> + n_bits = subs[i]->cmp.mask_n_bits;
>> + mask_index[n_bits] = MIN(mask_index[n_bits], i);
>> + }
>> + }
>> +
>> + /* Fill the gaps, so they point to an index with more bits. */
>> + for (i = max_n_bits; i > 0; i--) {
>> + mask_index[i] = MIN(mask_index[i], mask_index[i + 1]);
>> + }
>> +
>> + /* Find and eliminate supersets. */
>> + for (i = 0; i < n; i = next[i]) {
>> + if (!subs[i]) {
>> + continue;
>> + }
>> + /* 'subs' are sorted based on the number of bits in the mask.
>> + * For an expression to be a subset, it has to have more bits. */
>> + n_bits = subs[i]->cmp.mask_n_bits;
>> + if (mask_index[n_bits + 1] > n) {
>> + break;
>> + }
>> + for (j = mask_index[n_bits + 1]; j < n; j = next[j]) {
>> + struct expr *a = subs[i], *b = subs[j];
>> +
>> + ovs_assert(i != j);
>> + if (!b) {
>> + continue;
>> + }
>> +
>> + const unsigned long *a_value, *a_mask, *b_value, *b_mask;
>> + a_value = (unsigned long *) &a->cmp.value.be64[ofs];
>> + b_value = (unsigned long *) &b->cmp.value.be64[ofs];
>> + a_mask = (unsigned long *) &a->cmp.mask.be64[ofs];
>> + b_mask = (unsigned long *) &b->cmp.mask.be64[ofs];
>> +
>> + if (expr_bitmap_intersect_check(a_value, a_mask, b_value,
>> b_mask,
>> + bit_width)
>> + && bitmap_is_superset(b_mask, a_mask, bit_width)) {
>> + /* 'a' is the same expression with a smaller mask. */
>> + expr_destroy(subs[j]);
>> + subs[j] = NULL;
>> + }
>> + /* Shorten the path for the next round. */
>> + while (next[j] < n && !subs[next[j]]) {
>
> Maybe not a big deal, but why not just update the next[last] when the subs[j]
> is destroyed, which would be more straightforward and slightly more efficient?
I thought about changing on removal, but I thought it will require
keeping back references as well, i.e. have another 'prev' array,
which I didn't want. But you're right, if we can track the 'last'
non-NULL entry, then we can easily update on removal. I'll try that.
>
>> + next[j] = next[next[j]];
>> + }
>> + }
>> + }
>
> I think the algorithm is very smart. However, as you mentioned in the meeting
> today, the worst case of this nested loop can still be bad. For example, if
> we have an address set with 1000 addresses (not very big), but half of them
> are with the mask of /24, and the other half are with /32, without any
> overlapping, then it would be O(500 * 500) time complexity. This example may
> be too extreme, but in reality, I think we shouldn't be surprised to see
> someone using an address set with 10 CIDRs of /24 together with another 1000
> individual IPs, and the algorithm would make it 10 times slower than before.
> We should at least explicitly document this inefficient use of address sets
> and remind users to avoid the pitfalls, and in turn, document or optimize in
> CMS (k8s, openstack, ...) to avoid such usage.
We could document, sure. One thing worth mentioning is that we're
still talking about sub-millisecond times here. I did some tests
and normalization of the 10 CIDRs of /24 + 1000 individual IPs case
takes about 11 microseconds. The "too extreme" 500+500 case takes
700 microseconds, i.e. 0.7 ms. In comparison, if we do not look
for supersets, the same operations take 8 and 100 microseconds
respectively.
There is an obvious difference, especially in the 500+500 case.
I'm just not sure how much real impact we have as such big manually
created sets shouldn't really appear in way too many logical flows,
so we're probably talking about aggregated full recompute cost of
tens of milliseconds, maybe a hundred in extreme cases.
Just for fun I also tested the first patch from v2 and it is
horrible. :) It scores 0.3 and 26 ms respectively.
v3 is 30-40 times faster.
10+100 500+500
------- --------
main 8 us 100 us
v2 300 us 26000 us
v3 11 us 700 us
Here is what I used for testing:
---
diff --git a/tests/test-ovn.c b/tests/test-ovn.c
index d58350dcf..9917c38db 100644
--- a/tests/test-ovn.c
+++ b/tests/test-ovn.c
@@ -326,6 +326,20 @@ test_parse_expr__(int steps)
&ports);
}
if (steps > 2) {
+ const int n_iter = 10000;
+ struct expr **e = xmalloc(n_iter * sizeof *e);
+ for (int i = 0; i < n_iter; i++) {
+ e[i] = expr_clone(expr);
+ }
+ long long int start = time_usec();
+ for (int i = 0; i < n_iter; i++) {
+ e[i] = expr_normalize(e[i]);
+ }
+ printf("expr_normalize: %lld\n", (time_usec() - start) /
n_iter);
+ for (int i = 0; i < n_iter; i++) {
+ expr_destroy(e[i]);
+ }
+ free(e);
expr = expr_normalize(expr);
ovs_assert(expr_is_normalized(expr));
}
---
$ cat as.py
import netaddr
n24 = 10 # 500
n32 = 100 # 500
net24 = netaddr.IPNetwork('17.0.0.1/24')
net32 = netaddr.IPNetwork('10.0.0.1/32')
print('ip4.dst == {', end='')
for i in range(n24):
print(net24.next(i), ',', end='')
for i in range(n32):
if i == n32 - 1:
print(net32.next(i), end='')
else:
print(net32.next(i), ',', end='')
print('}')
---
$ python ./as.py > expr.in
$ ./tests/ovstest test-ovn normalize-expr < expr.in | grep normalize
expr_normalize: 11
>
> Alternatively, we may still restrict the algorithm for negation expr handling.
>
> @Mark Michelson <mailto:[email protected]> mentioned he will review it,
> too. So I'd like to hear his opinion.
>
> Thanks,
> Han
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