remove_entries_exceeding_cache_limit() is called after every network
response, but the cache is usually still under budget and nothing needs
to evict. Every one of those calls currently still runs the
window-function eviction SQL over the whole CacheIndex table just to
conclude there is nothing to do.
Short-circuit the call when the cache is already within its configured
size limit. To make that check cheap, maintain m_total_estimated_size
as a running total of the cache's estimated byte size, so the no-op
case becomes a single u64 compare and the DB is only touched when
there is real work.
Bookkeeping:
- Seed the total in CacheIndex::create() via a new
select_total_estimated_size statement (COALESCE(..., 0) so an empty
index returns 0 rather than NULL).
- Each Entry caches serialized_request_headers_size and
serialized_response_headers_size so we don't re-serialize to
recompute its footprint; Entry::estimated_size() centralizes the
arithmetic.
- create_entry() adds the new entry's size. Any row it displaces is
removed via DELETE ... RETURNING so the total stays accurate even
for entries that were never loaded into m_entries.
- remove_entry() and the bulk DELETE statements were extended with
the same RETURNING clause for the same reason.
- update_response_headers() shifts the total by the signed delta
between old and new serialized header size.
Also COALESCEs estimate_cache_size_accessed_since over an empty table
to 0 so callers don't have to special-case NULL.
create_entry() issues INSERT OR REPLACE in SQL, so the on-disk row is
correctly overwritten when a (cache_key, vary_key) pair is re-inserted.
But the in-memory m_entries vector was only appended to, leaving the
stale Entry alongside the new one. Subsequent find_entry() calls could
then return the old metadata even though the DB had moved on.
This just lets us create fewer cache directories during WPT. We do still
create cache entries on disk, so for WPT, we introduce an extra cache
key to prevent conflicts. There is an existing FIXME about this.
This adds a settings box to about:settings to allow users to limit the
disk cache size. This will override the default 5 GiB limit. We do not
automatically delete cache data if the new limit is suddenly less than
the used disk space; this will happen on the next request. This allows
multiple changes to the settings in a row without thrashing the cache.
In the future, we can add more toggles, such as disabling the disk
cache altogether.
Let's not attempt to cache entries that are excessively large. We limit
the cache data size to be 1/8 of the total disk cache limit, with a cap
of 256 MiB.
Rather than letting our disk cache grow unbounded, let's impose a limit
on the estimated total disk cache size. The limits chosen are vaguely
inspired by Chromium.
We impose a total disk cache limit of 5 GiB. Chromium imposes an overall
limit of 1.25 GiB; I've chosen more here because we currently cache
uncompressed data from cURL.
The limit is further restricted by the amount of available disk space,
which we just check once at startup (as does Chromium). We will choose a
percentage of the free space available on systems with limited space.
Our eviction errs on the side of simplicity. We will remove the least
recently accessed entries until the total estimated cache size does not
exceed our limit. This could potentially be improved in the future. For
example, if the next entry to consider is 40 MiB, and we only need to
free 1 MiB of space, we could try evicting slightly more recently used
entries. This would prevent evicting more than we need to.
Apparently, sqlite will fill this placeholder value in with NULL if we
do not pass a value. The query being executed here is:
UPDATE CacheIndex
SET last_access_time = ?
WHERE cache_key = ? AND vary_key = ?;
HeaderList::delete involves a Vector::remove_all_matching internally.
So if an exempt header appeared again later in the header list, we would
be accessing the name string of the previously deleted header.
We now partition the HTTP disk cache based on the Vary response header.
If a cached response contains a Vary header, we look for each of the
header names in the outgoing HTTP request. The outgoing request must
match every header value in the original request for the cache entry
to be used; otherwise, a new request will be issued, and a separate
cache entry will be created.
Note that we must now defer creating the disk cache file itself until we
have received the response headers. The Vary key is computed from these
headers, and affects the partitioned disk cache file name.
There are further optimizations we can make here. If we have a Vary
mismatch, we could find the best candidate cached response and issue a
conditional HTTP request. The content server may then respond with an
HTTP 304 if the mismatched request headers are actually okay. But for
now, if we have a Vary mismatch, we issue an unconditional request as
a purely correctness-oriented patch.
We need to store request headers in order to handle Vary mismatches.
(Note we should also be using BLOB for header storage in sqlite, as they
are not necessarily UTF-8.)
A future commit will format memory cache debug messages similarly to the
disk cache messages. To make it easy to read them both at a glance when
both debug flags are turned on, let's add a prefix to these messages.
We currently have two ongoing implementations of RFC 9111, HTTP caching.
In order to consolidate these, this patch moves the implementation from
RequestServer to LibHTTP for re-use within LibWeb.
2025-11-29 08:35:02 -05:00
Renamed from Services/RequestServer/Cache/CacheIndex.cpp (Browse further)
