Max out pagination


This document describes the best ways to calculate the size both in terms of bytes and handles of elements as they are added to a vector. This should be done in order to maximize the number of elements which can be batched at once while satisfying the kernel caps on channel writes.


To maximize throughput through a channel, it’s common to batch large responses as multiple vectors of things, for instance by using a pagination API. Since channels are capped at 64K bytes and 64 handles, comes the question of how many elements can be batched in the vector to max out the capacity (and yet, be just under the byte size and handle count thresholds).

The key reference document for the following is the FIDL wire format specification.

There are various examples that explain the best ways to max out pagination:

Bluetooth WatchPeers method

Consider the WatchPeers method of the fuchsia.bluetooth.sys.Access protocol, defined as:

WatchPeers() -> (vector<Peer> updated, vector<bt.PeerId> removed);

First, a request or response is preceded by a header, i.e. a fixed 16 bytes or sizeof(fidl_message_header_t) as defined here.

Each vector has a 16 bytes header sizeof(fidl_vector_t), followed by the content.

Since bt.PeerId is a struct{uint64} (defined here) it is a fixed 8 bytes, and therefore the removed vector’s content is the number of elements * 8 bytes.

Next, we need to estimate the size of Peer which is defined as a table. Tables are essentially a vector of envelopes, where each envelope then points to the field content. Estimating the size must be done in two steps:

  1. Determine the largest field ordinal used (a.k.a. max_set_ordinal)
  2. Determine the size of each present field

The size of Peer is then the table header -- i.e. sizeof(fidl_table_t), 16 bytes -- plus the largest set ordinal * envelope header (16 bytes) -- i.e. max_set_ordinal * sizeof(fidl_envelope_t) -- plus the total size of the content, that is, each present field’s content added.

Fields are relatively easy to size, many are primitives or wrappers thereof, hence result in 8 bytes (due to padding). The bt.Address field is also 8 bytes since it’s definition reduces to struct{uint8; array<uint8>:6}. The string field is a vector of bytes, i.e. sizeof(fidl_vector_t) + len(name), and padded to the nearest 8 bytes boundary.

Scenic Enqueue method

Consider the Enqueue method of the fuchsia.scenic.Session protocol, defined as:

Enqueue(vector<Command> cmds);

A request or response is preceded by a header, i.e. a fixed 16 bytes or sizeof(fidl_message_header_t) from zircon/fidl.h. Then, the vector has a 16 bytes header sizeof(fidl_vector_t), followed by the content of the vector which are the actual commands. As a result, before you account for the size of each individual command, there is a fixed size of 32 bytes.

A command is a union that has a 24 bytes header (i.e. sizeof(fidl_xunion_t)) followed by the content which is 8 bytes aligned.

The size of a Command union content depends on the variant selected. This example uses the input variant of type fuchsia.ui.input.Command.

The input variant (of the scenic command) is itself a union, which adds another 24 bytes header, followed by the content of that union, such as a send_pointer_input of type SendPointerInputCmd.

The simplified definition of SendPointerInputCmd and all transitively reachable types through this struct is provided below:

struct SendPointerInputCmd {
    uint32 compositor_id;
    PointerEvent pointer_event;

struct PointerEvent {
    uint64 event_time;
    uint32 device_id;
    uint32 pointer_id;
    PointerEventType type;
    PointerEventPhase phase;
    float32 x;
    float32 y;
    float32 radius_major;
    float32 radius_minor;
    uint32 buttons;

enum PointerEventType {
    // members elided

enum PointerEventPhase {
    // members elided

Both enums PointerEventType and PointerEventPhase default to an underlying representation of uint32. You can reduce the sizing of SendPointerInputCmd to the struct:

struct {
    uint32;   // 4 bytes, total 4
              // 4 bytes (padding due to increase in alignment), total 8
    uint64;   // 8 bytes, total 16
    uint32;   // 4 bytes, total 20
    uint32;   // 4 bytes, total 24
    uint32;   // 4 bytes, total 28
    uint32;   // 4 bytes, total 32
    float32;  // 4 bytes, total 36
    float32;  // 4 bytes, total 40
    float32;  // 4 bytes, total 44
    float32;  // 4 bytes, total 48
    uint32;   // 4 bytes, total 52

Therefore, the size of the SendPointerInputCmd struct is 52 bytes. For more information on struct sizing calculation, see The Lost Art of Structure Packing.

Now that you have sized all the pieces of a command, you add the total size:

  • Header of fuchsia.ui.scenic.Command: 24 bytes, i.e sizeof(fidl_xunion_t)
  • Content with variant input:
    • Header of fuchsia.ui.input.Command: 24 bytes, i.e sizeof(fidl_xunion_t)
    • Content with variant set_hard_keyboard_delivery:
      • Struct SendPointerInputCmd: 52 bytes
      • Padding to align to 8 bytes: 4 bytes

This results in a total size of 104 bytes.