> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-parallel-read-in-order-multi-part.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# webassembly_udf_* session settings

> ClickHouse session settings in the webassembly_udf_* generated group.

export const ExperimentalBadge = () => {
  return <a href="https://clickhouse.com/docs/reference/settings/beta-and-experimental-features#experimental-features" className="experimentalBadge">
            <div className="experimentalIcon">
            <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
                <path strokeWidth="1.25" d="M5.5 2H10.5" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
                <path strokeWidth="1.25" d="M9.50015 2V6.19625L13.4283 12.7425C13.4738 12.8183 13.4985 12.9049 13.4996 12.9934C13.5008 13.0818 13.4785 13.169 13.435 13.246C13.3914 13.323 13.3283 13.3871 13.2519 13.4317C13.1755 13.4764 13.0886 13.4999 13.0002 13.5H3.00015C2.91164 13.5 2.8247 13.4766 2.74822 13.432C2.67174 13.3874 2.60847 13.3233 2.56487 13.2463C2.52126 13.1693 2.49889 13.082 2.50004 12.9935C2.50119 12.905 2.52582 12.8184 2.5714 12.7425L6.50015 6.19625V2" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
                <path strokeWidth="1.25" d="M4.47656 9.56754C5.30344 9.41254 6.47656 9.47942 7.99969 10.25C10.0153 11.2707 11.4216 11.0569 12.2184 10.7282" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
            </svg>
        </div>
            Experimental feature
        </a>;
};

export const VersionHistory = ({rows = []}) => {
  if (rows.length === 0) {
    return null;
  }
  const headers = ["Version", "Default value", "Comment"];
  const border = "1px solid rgba(128, 128, 128, 0.3)";
  const cell = {
    border,
    padding: "0.25rem 0.5rem",
    textAlign: "start",
    verticalAlign: "top"
  };
  return <details className="not-prose" style={{
    border,
    borderRadius: "0.5rem",
    margin: "0.5rem 0",
    padding: "0.5rem 0.75rem",
    fontSize: "0.8125rem",
    lineHeight: "1.125rem"
  }}>
      <summary style={{
    cursor: "pointer",
    fontWeight: 600,
    opacity: 0.72
  }}>
        Version history
      </summary>
      <table style={{
    borderCollapse: "collapse",
    width: "100%",
    margin: "0.5rem 0 0"
  }}>
        <thead>
          <tr>
            {headers.map(header => <th key={header} style={{
    ...cell,
    fontWeight: 600,
    opacity: 0.72
  }}>
                {header}
              </th>)}
          </tr>
        </thead>
        <tbody>
          {rows.map((row, row_index) => <tr key={row.id ?? row_index}>
              {(row.items ?? []).map((item, item_index) => <td key={item_index} style={{
    ...cell,
    overflowWrap: "anywhere"
  }}>
                  {item?.label}
                </td>)}
            </tr>)}
        </tbody>
      </table>
    </details>;
};

export const SettingsInfoBlock = ({type, default_value, changeable_without_restart}) => {
  return <div className="not-prose" style={{
    display: "flex",
    flexWrap: "wrap",
    alignItems: "baseline",
    columnGap: "0.5rem",
    rowGap: "0.125rem",
    margin: "0.375rem 0",
    fontSize: "0.8125rem",
    lineHeight: "1.125rem"
  }}>
      <div style={{
    fontWeight: 600,
    opacity: 0.72
  }}>Type</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{type}</div>
      <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>Default</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{default_value}</div>
      {changeable_without_restart && <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>
          Changeable without restart
        </div>}
      {changeable_without_restart && <div style={{
    overflowWrap: "anywhere"
  }}>
          {changeable_without_restart}
        </div>}
    </div>;
};

These settings are available in [system.settings](/reference/system-tables/settings) and are autogenerated from [source](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp).

<h2 id="webassembly_udf_input_split_memory_ratio">
  webassembly\_udf\_input\_split\_memory\_ratio
</h2>

<ExperimentalBadge />

<SettingsInfoBlock type="Float" default_value="0.5" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.9"},{"label": "0.5"},{"label": "New setting controlling the fraction of a WebAssembly UDF instance's linear memory that one call's serialized input may occupy, which also enables the dynamic splitting of that input by its serialized size; `compatibility` below 26.9 sets it to 0 and restores the previous behavior, where `webassembly_udf_max_input_block_size = 0` meant one call per pipeline block."}]}]} />

Fraction of a WebAssembly UDF instance's linear memory that one call's serialized input may occupy. Must be at least 0 and at most 1; the default leaves the other half to the guest for its own working set beside the input buffer.

The margin below 1 is what makes the batching safe: the guest's own data, stack and allocator share that memory and are invisible to the host. A ratio close to 1 leaves nothing for them, so a call sized against it can still fail inside the guest's allocator.

Read only for `ABI BUFFERED_V1`, and it sizes the calls only while `webassembly_udf_max_input_block_size` is 0 - a non-zero block size caps the rows per call instead.

A batch is never taken below a single row, so a row whose own serialized size is past the budget is passed on its own, and one too large for the module's linear memory fails inside the guest's allocator.

Set to 0 to leave the input unsplit: the whole pipeline block is passed in one call.

<h2 id="webassembly_udf_max_fuel">
  webassembly\_udf\_max\_fuel
</h2>

<ExperimentalBadge />

<SettingsInfoBlock type="UInt64" default_value="100000" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "100000"},{"label": "New setting to limit CPU instructions (fuel) per WebAssembly UDF instance execution."}]}]} />

Fuel limit per WebAssembly UDF instance execution. Each WebAssembly instruction consumes some amount of fuel. The value is scaled by 1024 before being passed to the runtime, so `webassembly_udf_max_fuel = 1` corresponds to approximately 1024 fuel units. Set to 0 for no finite limit. Applies only to functions whose per-function setting `webassembly_udf_enable_fuel` is true, which is the default.

<h2 id="webassembly_udf_max_input_block_size">
  webassembly\_udf\_max\_input\_block\_size
</h2>

<ExperimentalBadge />

<SettingsInfoBlock type="UInt64" default_value="0" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "0"},{"label": "New setting to limit input block size for WebAssembly UDFs."}]}]} />

Maximum number of rows passed to a WebAssembly UDF in a single call. A non-zero value caps the rows per call and applies to every ABI.

Set to 0 to size the calls by their serialized input instead. That applies to `ABI BUFFERED_V1` alone, the only ABI that serializes a whole input block into guest memory: its blocks are split so that a call's input stays within `webassembly_udf_input_split_memory_ratio` of the module's linear memory. `ROW_DIRECT` passes its arguments as WebAssembly values and `ASSEMBLYSCRIPT` builds one object per row, so neither has a serialized input to size a call by, and 0 leaves their pipeline block whole.

<h2 id="webassembly_udf_max_instances">
  webassembly\_udf\_max\_instances
</h2>

<ExperimentalBadge />

<SettingsInfoBlock type="UInt64" default_value="32" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "32"},{"label": "New setting to limit the number of parallel WebAssembly UDF instances per function."}]}]} />

Maximum number of WebAssembly UDF instances that can run in parallel per function.

<h2 id="webassembly_udf_max_memory">
  webassembly\_udf\_max\_memory
</h2>

<ExperimentalBadge />

<SettingsInfoBlock type="UInt64" default_value="134217728" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "134217728"},{"label": "New setting to limit memory per WebAssembly UDF instance."}]}]} />

Memory limit in bytes per WebAssembly UDF instance.