Skip to content

Commit

Permalink
json: Store short strings in-place.
Browse files Browse the repository at this point in the history 
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 by up to 10% while serializing large
OVN databases.

Signed-off-by: Ilya Maximets <[email protected]>
  • Loading branch information
@igsilya
igsilya committed Dec 16, 2024
1 parent 1bce015 commit 2671d43
Show file tree
Hide file tree
Showing 5 changed files with 64 additions and 19 deletions.
14 changes: 13 additions & 1 deletion include/openvswitch/json.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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. */
};
};

Expand Down
48 changes: 36 additions & 12 deletions lib/json.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -179,29 +179,41 @@ 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_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;
}

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;
}

Expand Down Expand Up @@ -346,14 +358,16 @@ 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;
ovs_assert(json->storage_type == JSON_STRING_DYNAMIC);
return json->str_ptr;
}

struct json_array *
Expand Down Expand Up @@ -408,8 +422,13 @@ json_destroy__(struct json *json, bool yield)
break;

case 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:
Expand Down Expand Up @@ -577,9 +596,11 @@ json_hash(const struct json *json, size_t basis)
return json_hash_array(&json->array, basis);

case JSON_STRING:
case JSON_SERIALIZED_OBJECT:
return hash_string(json_string(json), basis);

case JSON_SERIALIZED_OBJECT:
return hash_string(json->str_ptr, basis);

case JSON_NULL:
case JSON_FALSE:
case JSON_TRUE:
Expand Down Expand Up @@ -653,9 +674,11 @@ json_equal(const struct json *a, const struct json *b)
return json_equal_array(&a->array, &b->array);

case JSON_STRING:
case JSON_SERIALIZED_OBJECT:
return !strcmp(json_string(a), json_string(b));

case JSON_SERIALIZED_OBJECT:
return !strcmp(a->str_ptr, b->str_ptr);

case JSON_NULL:
case JSON_FALSE:
case JSON_TRUE:
Expand Down Expand Up @@ -989,7 +1012,8 @@ 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);
}
Expand Down Expand Up @@ -1053,7 +1077,7 @@ struct json *
json_from_serialized_object(const struct json *json)
{
ovs_assert(json->type == JSON_SERIALIZED_OBJECT);
return json_from_string(json_string(json));
return json_from_string(json->str_ptr);
}

/* Reads the file named 'file_name', parses its contents as a JSON object or
Expand Down Expand Up @@ -1649,7 +1673,7 @@ json_serialize(const struct json *json, struct json_serializer *s)
break;

case JSON_SERIALIZED_OBJECT:
ds_put_cstr(ds, json_string(json));
ds_put_cstr(ds, json->str_ptr);
break;

case JSON_N_TYPES:
Expand Down
6 changes: 4 additions & 2 deletions python/ovs/db/data.py
View file
Original file line number Diff line number Diff line change
Expand Up @@ -573,7 +573,8 @@ def cDeclareDatum(self, name):
% (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)]
Expand All @@ -592,7 +593,8 @@ def cDeclareDatum(self, name):
% (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)]
Expand Down
6 changes: 5 additions & 1 deletion tests/test-json.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -100,11 +100,15 @@ test_json_equal(const struct json *a, const struct json *b,
return;

case JSON_STRING:
case JSON_SERIALIZED_OBJECT:
ovs_assert(json_string(a) != json_string(b));
ovs_assert(!strcmp(json_string(a), json_string(b)));
return;

case JSON_SERIALIZED_OBJECT:
ovs_assert(a->str_ptr != b->str_ptr);
ovs_assert(!strcmp(a->str_ptr, b->str_ptr));
return;

case JSON_NULL:
case JSON_FALSE:
case JSON_TRUE:
Expand Down
9 changes: 6 additions & 3 deletions tests/test-ovsdb.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2412,10 +2412,13 @@ 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;
Expand Down

0 comments on commit 2671d43

Please sign in to comment.