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Technical Briefing: Parallel ESI for Varnish 6.0 LTS via vmod-pesi
The Problem: Sequential ESI Fragment Assembly
Stock open-source Varnish processes ESI includes sequentially. A page containing two uncached fragments that each take 2 seconds to render therefore takes over 4 seconds to assemble — the total is the sum of fragment latencies, not the maximum. This is a structural limitation of the open-source daemon, not a configuration issue.
The Implementation: A Backport of libvdp-pesi
GetPageSpeed backported Uplex’s open-source libvdp-pesi v0.2.7 — which upstream targets Varnish 6.2+ — to Varnish 6.0 LTS, packaging it as vmod-pesi in the getpagespeed-extras-varnish60 repository channel.
Key packaging properties:
- Built against
varnish-6.0.18exactly, so the module and daemon cannot drift apart. - Distributed as a GetPageSpeed backport of Uplex’s BSD-2-Clause code. It is not supported or endorsed by Uplex.
- Because Varnish 6.0 lacks the
resp.filtersmechanism upstream relies on, activation was re-implemented at the transport layer.
Installation
On RHEL 8+ derivatives: install the release RPM and dnf-plugins-core, enable getpagespeed-extras-varnish60, then:
dnf -y install varnish vmod-pesi
On EL7: use the yum/yum-utils equivalents.
Production use requires a GetPageSpeed subscription (which also covers 100+ NGINX modules).
Activation
ESI pages are marked as usual with set beresp.do_esi = true. Then add to VCL:
import pesi;
and call pesi.activate() in vcl_deliver. No changes are needed to ESI markup, backends, or purging logic. The unconditional call is a no-op for responses without ESI data.
Verified Behavior
On a clean Rocky Linux 10 machine, two ESI includes each backed by a 2-second backend showed:
- Sequential (stock): ~2s + 2s
- Parallel (
vmod-pesi): max(2s, 2s)
Response bodies were confirmed byte-identical via cmp. Fully cached pages are sub-millisecond either way.
Beyond total time, the level-0 thread can start sending completed page parts to the client while other fragments assemble, improving time to first byte.
Supported Platforms
EL7–10 (RHEL, Rocky, Alma, CentOS Stream, Oracle), Amazon Linux 2 and 2023, Fedora 43 and 44, SLES 16 — on x86_64 and aarch64.
Rules and Caveats
pesi.activate()must run for the top page and its includes; the unconditional call covers this.- Mixing parallel and stock delivery across levels of one ESI tree is unsupported and can crash the worker process.
- Anything affecting ESI, gzip, or Range processing (e.g., touching
req.http.Accept-Encoding) must happen before thepesi.activate()call.
Tuning Knobs (pesi.set() in vcl_deliver)
| Directive | Effect |
|---|---|
pesi.set(serial, true) |
Processes the next include level in the current thread. Sensible for deeper levels where includes are cache hits; not recommended at level 0. |
pesi.set(thread, false) |
Uses a new thread only when immediately available. Default true is right for almost everyone. |
pesi.pool() / pesi.workspace_prealloc() |
Tune internal memory pooling; defaults are fine. |
Fragment-heavy sites should sanity-check thread_pool_min / thread_pool_max.
Range Behavior
Non-ESI cached files still return 206 Partial Content with exact bytes under the new VCL, pinned by a regression test. ESI responses are delivered whole with 200 regardless of Range — same as stock Varnish.
Magento 2 Fit
Magento 2’s Varnish integration targets the 6.0 LTS line and renders private/short-lived blocks as ESI includes. Adding import pesi; and pesi.activate() to the Magento-generated VCL parallelizes them without touching the store. The same approach applies to WordPress behind Varnish, portals with personalized sidebars, and dashboards mixing cached and live widgets.
Porting Notes
On Fedora 43+ and SLES 16, gcc 15 defaults to C23, where bool became a reserved keyword and broke runtime VCL compilation. The optional VCL argument was renamed; because callers are positional, there is no change for users.
Testing
The complete upstream varnishtest suite passes against Varnish 6.0.18 on every shipped distro and architecture, plus added regression tests for:
- A nested gzipped ESI delivery-ordering issue found during porting.
- Stock Range behavior on non-ESI responses.
NGINX Alternative
For a single cache tier without Varnish, the Pro-tier nginx-module-esi brings native ESI with parallel fragment fetching to NGINX. Both paths are covered by the same subscription.
Read the full article: Parallel ESI for Varnish 6.0 LTS: vmod-pesi Now Packaged
