Reducing memory consumption in librsvg, part 2: SpecifiedValues

Translations: es - Tags: gnome, librsvg, performance, rust

To continue with last time's topic, let's see how to make librsvg's DOM nodes smaller in memory. Since that time, there have been some changes to the code; that is why in this post some of the type names are different from last time's.

Every SVG element is represented with this struct:

pub struct Element {
    element_type: ElementType,
    element_name: QualName,
    id: Option<String>,
    class: Option<String>,
    specified_values: SpecifiedValues,
    important_styles: HashSet<QualName>,
    result: ElementResult,
    transform: Transform,
    values: ComputedValues,
    cond: bool,
    style_attr: String,
    element_impl: Box<dyn ElementTrait>,

The two biggest fields are the ones with types SpecifiedValues and ComputedValues. These are the sizes of the whole Element struct and those two types:

sizeof Element: 1808
sizeof SpecifiedValues: 824
sizeof ComputedValues: 704

In this post, we'll reduce the size of SpecifiedValues.

What is SpecifiedValues?

If we have an element like this:

<circle cx="10" cy="10" r="10" stroke-width="4" stroke="blue"/>

The values of the style properties stroke-width and stroke get stored in a SpecifiedValues struct. This struct has a bunch of fields, one for each possible style property:

pub struct SpecifiedValues {
    baseline_shift:              SpecifiedValue<BaselineShift>,
    clip_path:                   SpecifiedValue<ClipPath>,
    clip_rule:                   SpecifiedValue<ClipRule>,
    /// ...
    stroke:                      SpecifiedValue<Stroke>,
    stroke_width:                SpecifiedValue<StrokeWidth>,
    /// ...

Each field is a SpecifiedValue<T> for the following reason. In CSS/SVG, a style property can be unspecified, or it can have an inherit value to force the property to be copied from the element's parent, or it can actually have a specified value. Librsvg represents these as follows:

pub enum SpecifiedValue<T>
    T: // some trait bounds here

Now, SpecifiedValues has a bunch of fields, 47 of them to be exact — one for each of the style properties that librsvg supports. That is why SpecifiedValues has a size of 824 bytes; it is the largest sub-structure within Element, and it would be good to reduce its size.

Not all properties are specified

Let's go back to the chunk of SVG from above:

<circle cx="10" cy="10" r="10" stroke-width="4" stroke="blue"/>

Here we only have two specified properties, so the stroke_width and stroke fields of SpecifiedValues will be set as SpecifiedValue::Specified(something) and all the other fields will be left as SpecifiedValue::Unspecified.

It would be good to store only complete values for the properties that are specified, and just a small flag for unset properties.

Another way to represent the set of properties

Since there is a maximum of 47 properties per element (or more if librsvg adds support for extra ones), we can have a small array of 47 bytes. Each byte contains the index within another array that contains only the values of specified properties, or a sentinel value for properties that are unset.

First, I made an enum that fits in a u8 for all the properties, plus the sentinel value, which also gives us the total number of properties. The #[repr(u8)] guarantees that this enum fits in a byte.

enum PropertyId {
    // ...
    UnsetProperty, // the number of properties and also the sentinel value

Also, since before these changes there was the following monster to represent "which property is this" plus the property's value:

pub enum ParsedProperty {
    // ...

I changed the definition of SpecifiedValues to have two arrays, one to store which properties are specified, and another only with the values for the properties that are actually specified:

pub struct SpecifiedValues {
    indices: [u8; PropertyId::UnsetProperty as usize],
    props: Vec<ParsedProperty>,

There is a thing that is awkward in Rust, or which I haven't found how to solve in a nicer way: given a ParsedProperty, find the corresponding PropertyId for its discriminant. I did the obvious thing:

impl ParsedProperty {
    fn get_property_id(&self) -> PropertyId {
        use ParsedProperty::*;

        match *self {
            BaselineShift(_) => PropertyId::BaselineShift,
            ClipPath(_)      => PropertyId::ClipPath,
            ClipRule(_)      => PropertyId::ClipRule,
            Color(_)         => PropertyId::Color,
            // ...


First, we want to initialize an empty SpecifiedValues, where every element of the the indices array is set to the sentinel value that means that the corresponding property is not set:

impl Default for SpecifiedValues {
    fn default() -> Self {
        SpecifiedValues {
            indices: [PropertyId::UnsetProperty.as_u8(); PropertyId::UnsetProperty as usize],
            props: Vec::new(),

That sets the indices field to an array full of the same PropertyId::UnsetProperty sentinel value. Also, the props array is empty; it hasn't even had a block of memory allocated for it yet. That way, SVG elements without style properties don't use any extra memory.

Which properties are specified and what are their indices?

Second, we want a function that will give us the index in props for some property, or that will tell us if the property has not been set yet:

impl SpecifiedValues {
    fn property_index(&self, id: PropertyId) -> Option<usize> {
        let v = self.indices[id.as_usize()];

        if v == PropertyId::UnsetProperty.as_u8() {
        } else {
            Some(v as usize)

(If someone passes id = PropertyId::UnsetProperty, the array access to indices will panic, which is what we want, since that is not a valid property id.)

Change a property's value

Third, we want to set the value of a property that has not been set, or change the value of one that was already specified:

impl SpecifiedValues {
    fn replace_property(&mut self, prop: &ParsedProperty) {
        let id = prop.get_property_id();

        if let Some(index) = self.property_index(id) {
            self.props[index] = prop.clone();
        } else {
            let pos = self.props.len() - 1;
            self.indices[id.as_usize()] = pos as u8;

In the first case in the if, the property was already set and we just replace its value. In the second case, the property was not set; we add it to the props array and store its resulting index in indices.



sizeof Element: 1808
sizeof SpecifiedValues: 824


sizeof Element: 1056
sizeof SpecifiedValues: 72

The pathological file from the last time used 463,412,720 bytes in memory before these changes. After the changes, it uses 314,526,136 bytes.

I also measured memory consumption for a normal file, in this case one with a bunch of GNOME's symbolic icons. The old version uses 17 MB; the new version only 13 MB.

How to keep fine-tuning this

For now, I am satisfied with SpecifiedValues, although it could still be made smaller:

  • The crate tagged-box converts an enum like ParsedProperty into an enum-of-boxes, and codifies the enum's discriminant into the box's pointer. This way each variant occupies the minimum possible memory, although in a separately-allocated block, and the container itself uses only a pointer. I am not sure if this is worth it; each ParsedProperty is 64 bytes, but the flat array props: Vec<ParsedProperty> is very appealing in a single block of memory. I have not checked the sizes of each individual property to see if they vary a lot among them.

  • Look for a crate that lets us have the properties in a single memory block, a kind of arena with variable types. This can be implemented with a bit of unsafe, but one has to be careful with the alignment of different types.

  • The crate enum_set2 represents an array of field-less enums as a compact bit array. If we changed the representation of SpecifiedValue, this would reduce the indices array to a minimum.

If someone wants to dedicate some time to implement and measure this, I would be very grateful.

Next steps

According to Massif, the next thing is to keep making Element smaller. The next thing to shrink is ComputedValues. The obvious route is to do exactly the same as I did for SpecifiedValues. I am not sure if it would be better to try to share the style structs between elements.