Virtuoso supports HTTP 1.1 Chunking Encoding which allows Virtuoso to send the user agent chunks of output as the page is still processing. Chunking is enabled by calling http_flush(1) within the VSP page. Chunks are sent for every 4k worth of output generated. Chunked mode requires the following conditions:

no "Content-Length" header sent to the client using http_header()
no "Content-Encoding" header sent to the client using http_header()
use http_xslt() is not permitted
The client supports HTTP 1.1

Failing these conditions, http_flush(1) will be a No-Operation.

Chunked mode is not supported for static content.