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json: Store short strings in-place.

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The 'struct json' contains a union and the largest element of that
union is 'struct json_array', which takes 24 bytes.  It means, that a
lot of space in this structure remains unused whenever the type is not
JSON_ARRAY.

For example, the 'string' pointer used for JSON_STRING only takes 8
bytes on a 64-bit system leaving 24 - 8 = 16 bytes unused.  There is
also a 4-byte hole between the 'type' and the 'count'.

A pretty common optimization technique for storing strings is to
store short ones in place of the pointer and only allocate dynamically
the larger strings that do not fit.  In our case, we have even larger
space of 24 bytes to work with.  So, we could use all 24 bytes to
store the strings (23 string bytes + '\0') and use the 4 byte unused
space outside the union to store the storage type.

This approach should allow us to save on memory allocation for short
strings and also save on accesses to them, as the content will fit
into the same cache line as the 'struct json' itself.

In practice, large OVN databases tend to operate with quite large
strings.  For example, all the logical flow matches and actions in
OVN Southbound database would not fit.  However, this approach still
allows to improve performance with large OVN databases.

With 350MB OVN Northbound database with 12M atoms:

                         Before        After       Improvement
 ovsdb-client dump      18.6 sec      16.6 sec       10.7 %
 Compaction             14.0 sec      13.4 sec        4.2 %
 Memory usage (RSS)     2.28 GB       2.05 GB        10.0 %

With 615MB OVN Southbound database with 23M atoms:

                         Before        After       Improvement
 ovsdb-client dump      46.1 sec      43.7 sec        5.2 %
 Compaction             34.8 sec      32.5 sec        6.6 %
 Memory usage (RSS)     5.29 GB       4.80 GB         9.3 %

In the results above, 'ovsdb-client dump' is measuring how log it
takes for the server to prepare and send a reply, 'Memory usage (RSS)'
reflects the RSS of the ovsdb-server after loading the full database.
ovn-heater tests report similar reduction in CPU and memory usage
on heavy operations like compaction.

Acked-by: Mike Pattrick <mkp@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
This commit is contained in:
Ilya Maximets 2025-06-24 21:54:32 +02:00
parent 6c48b29f52
commit 9669b50f56
4 changed files with 48 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
include/openvswitch/json.h
View File

@ -63,16 +63,28 @@ struct json_array {
struct json **elems;
};
/* Maximum string length that can be stored inline ('\0' is not included). */
#define JSON_STRING_INLINE_LEN (sizeof(struct json_array) - 1)
enum json_storage_type {
JSON_STRING_DYNAMIC = 0, /* JSON_STRING is stored via 'str_ptr'. */
JSON_STRING_INLINE, /* JSON_STRING is stored in 'str' array. */
};
/* A JSON value. */
struct json {
enum json_type type;
enum json_storage_type storage_type;
size_t count;
union {
struct shash *object; /* Contains "struct json *"s. */
struct json_array array;
long long int integer;
double real;
char *string; /* JSON_STRING or JSON_SERIALIZED_OBJECT. */
union {
char str[JSON_STRING_INLINE_LEN + 1];
char *str_ptr;
}; /* JSON_STRING or JSON_SERIALIZED_OBJECT. */
};
};

34
lib/json.c
View File

@ -179,14 +179,26 @@ struct json *
json_string_create_nocopy(char *s)
{
struct json *json = json_create(JSON_STRING);
json->string = s;
json->storage_type = JSON_STRING_DYNAMIC;
json->str_ptr = s;
return json;
}
struct json *
json_string_create(const char *s)
{
return json_string_create_nocopy(xstrdup(s));
struct json *json = json_create(JSON_STRING);
size_t length = strlen(s);
if (length <= JSON_STRING_INLINE_LEN) {
json->storage_type = JSON_STRING_INLINE;
memcpy(json->str, s, length);
json->str[length] = '\0';
} else {
json->storage_type = JSON_STRING_DYNAMIC;
json->str_ptr = xmemdup0(s, length);
}
return json;
}
struct json *json_string_create_uuid(const struct uuid *uuid)
@ -198,7 +210,7 @@ struct json *
json_serialized_object_create(const struct json *src)
{
struct json *json = json_create(JSON_SERIALIZED_OBJECT);
json->string = json_to_string(src, JSSF_SORT);
json->str_ptr = json_to_string(src, JSSF_SORT);
return json;
}
@ -206,7 +218,7 @@ struct json *
json_serialized_object_create_with_yield(const struct json *src)
{
struct json *json = json_create(JSON_SERIALIZED_OBJECT);
json->string = json_to_string(src, JSSF_SORT | JSSF_YIELD);
json->str_ptr = json_to_string(src, JSSF_SORT | JSSF_YIELD);
return json;
}
@ -357,14 +369,15 @@ const char *
json_string(const struct json *json)
{
ovs_assert(json->type == JSON_STRING);
return json->string;
return json->storage_type == JSON_STRING_DYNAMIC
? json->str_ptr : json->str;
}
const char *
json_serialized_object(const struct json *json)
{
ovs_assert(json->type == JSON_SERIALIZED_OBJECT);
return json->string;
return json->str_ptr;
}
struct json_array *
@ -419,11 +432,13 @@ json_destroy__(struct json *json, bool yield)
break;
case JSON_STRING:
free(json->string);
if (json->storage_type == JSON_STRING_DYNAMIC) {
free(json->str_ptr);
}
break;
case JSON_SERIALIZED_OBJECT:
free(json->string);
free(json->str_ptr);
break;
case JSON_NULL:
@ -1007,7 +1022,8 @@ json_string_escape(const char *in, struct ds *out)
{
struct json json = {
.type = JSON_STRING,
.string = CONST_CAST(char *, in),
.storage_type = JSON_STRING_DYNAMIC,
.str_ptr = CONST_CAST(char *, in),
};
json_to_ds(&json, 0, out);
}

6
python/ovs/db/data.py
View File

@ -573,7 +573,8 @@ class Datum(object):
% (name, n)]
for key in sorted(self.values):
s += [' { .type = JSON_STRING, '
'.string = "%s", .count = 2 },'
'.storage_type = JSON_STRING_DYNAMIC, '
'.str_ptr = "%s", .count = 2 },'
% escapeCString(key.value)]
s += ["};"]
s += ["static union ovsdb_atom %s_keys[%d] = {" % (name, n)]
@ -592,7 +593,8 @@ class Datum(object):
% (name, n)]
for k, v in sorted(self.values):
s += [' { .type = JSON_STRING, '
'.string = "%s", .count = 2 },'
'.storage_type = JSON_STRING_DYNAMIC, '
'.str_ptr = "%s", .count = 2 },'
% escapeCString(v.value)]
s += ["};"]
s += ["static union ovsdb_atom %s_values[%d] = {" % (name, n)]

9
tests/test-ovsdb.c
View File

@ -2413,10 +2413,13 @@ substitute_uuids(struct json *json, const struct ovsdb_symbol_table *symtab)
if (json->type == JSON_STRING) {
const struct ovsdb_symbol *symbol;
symbol = ovsdb_symbol_table_get(symtab, json->string);
symbol = ovsdb_symbol_table_get(symtab, json_string(json));
if (symbol) {
free(json->string);
json->string = xasprintf(UUID_FMT, UUID_ARGS(&symbol->uuid));
if (json->storage_type == JSON_STRING_DYNAMIC) {
free(json->str_ptr);
}
json->storage_type = JSON_STRING_DYNAMIC;
json->str_ptr = xasprintf(UUID_FMT, UUID_ARGS(&symbol->uuid));
}
} else if (json->type == JSON_ARRAY) {
size_t i;