Bug 189760 – Resource Load Statistics: Relax cap on partitioned cache max age if reloads result in the same bytes

• Component: WebKit Misc.
• Status: RESOLVED FIXED

[John Wilander]

We should add logic to achieve the following:
1. Set cache max age cap for prevalent resources to N days.
2. Start every new cache entry with a max age cap multiplier M = 1.
3. When a cache entry for a prevalent resource reaches the age of N*M days, discard it, but hold on to its checksum and M.
4. If the reload results in the same checksum, increase M according to some scheme such as linear or doubling, until it reaches a year.

[Radar WebKit Bug Importer]

<rdar://problem/44612242>

[John Wilander]

Created attachment 352933 [details]
Patch

[John Wilander]

Antti and Youenn, I wanted to get some feedback from you on this. The Changelog has an explanation of what's in the patch.

[John Wilander]

It may very well be that we need to persist what's in the m_forcedReloadEntries HashMap.

[John Wilander]

Created attachment 352935 [details]
Patch

[John Wilander]

Fixed a non-Cocoa build issue.

[John Wilander]

Created attachment 352938 [details]
Patch

[John Wilander]

Created attachment 352939 [details]
Patch

[John Wilander]

Created attachment 352987 [details]
Patch

[John Wilander]

Corrected to the code to use ResourceResponseBase::isRedirection() instead of ResourceResponseBase::isRedirected(). Fleshed out test case to cover this functionality.

[Antti Koivisto]

Comment on attachment 352987 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=352987&action=review

> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:281
> +#if ENABLE(RESOURCE_LOAD_STATISTICS)

Why is this feature called RESOURCE_LOAD_STATISTICS? It is confusing since it is not about collecting statistics.

> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:284
> +        auto verificationEntry = m_forcedReloadEntries.take(identifier);

This mechanism only works if the network process stays alive so that m_forcedReloadEntries is around when the cache entry is rewritten. Why is this a good assumption? Especially on iOS the browser may get closed a lot. Should we persist the data in m_forcedReloadEntries, maybe to the cache entry itself?

The size of the m_forcedReloadEntries is a potential concern. Wouldn't it eventually contain entries for pretty much every resource stored in a session, which could be a very large number.

> Source/WebKit/NetworkProcess/cache/NetworkCacheEntry.cpp:103
>      encoder.encodeChecksum();
>  
> +#if ENABLE(RESOURCE_LOAD_STATISTICS)
> +    if (m_maxAgeCap)
> +        encoder << m_maxAgeCap;
> +#endif

This needs to be encoded before the checksum.

Since you are encoding an optional, this should be done unconditionally (no if branch).

> Source/WebKit/NetworkProcess/cache/NetworkCacheEntry.cpp:113
> +    return { m_key, m_timeStamp, header, body, { }, std::nullopt };

std::nullopt -> { } for consistency.

> Source/WebKit/NetworkProcess/cache/NetworkCacheEntry.cpp:153
> +#if ENABLE(RESOURCE_LOAD_STATISTICS)
> +    // Optional max age cap.
> +    decoder.decode(entry->m_maxAgeCap);
> +#endif

Similarly decoding should happen before the checksum check. 

You'll should check the return value and return nullptr if decoding fails.

[youenn fablet]

Comment on attachment 352987 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=352987&action=review

>> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:284
>> +        auto verificationEntry = m_forcedReloadEntries.take(identifier);
> 
> This mechanism only works if the network process stays alive so that m_forcedReloadEntries is around when the cache entry is rewritten. Why is this a good assumption? Especially on iOS the browser may get closed a lot. Should we persist the data in m_forcedReloadEntries, maybe to the cache entry itself?
> 
> The size of the m_forcedReloadEntries is a potential concern. Wouldn't it eventually contain entries for pretty much every resource stored in a session, which could be a very large number.

NetworkCache is a long living object so it might be better to move the state handling to NetworkResourceLoader (the instance or some of its completion handlers).
That way, there will be no risk of leaking memory by having forever stored entries in m_forcedReloadEntries (say a load is cancelled).

[John Wilander]

(In reply to youenn fablet from comment #12)
> Comment on attachment 352987 [details]
> Patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=352987&action=review
> 
> >> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:284
> >> +        auto verificationEntry = m_forcedReloadEntries.take(identifier);
> > 
> > This mechanism only works if the network process stays alive so that m_forcedReloadEntries is around when the cache entry is rewritten. Why is this a good assumption? Especially on iOS the browser may get closed a lot. Should we persist the data in m_forcedReloadEntries, maybe to the cache entry itself?
> > 
> > The size of the m_forcedReloadEntries is a potential concern. Wouldn't it eventually contain entries for pretty much every resource stored in a session, which could be a very large number.
> 
> NetworkCache is a long living object so it might be better to move the state
> handling to NetworkResourceLoader (the instance or some of its completion
> handlers).
> That way, there will be no risk of leaking memory by having forever stored
> entries in m_forcedReloadEntries (say a load is cancelled).

That is a good suggestion. I'll look into it.

Anything else on this patch?

[John Wilander]

Created attachment 353747 [details]
Patch

[John Wilander]

I've moved the logic out to the NetworkResourceLoader as suggested. I've also removed the global state.

I could not rely on the existing validation infrastructure since redirects are not handled by there. Instead I tie into the special functions for redirects.

[John Wilander]

Created attachment 353749 [details]
Patch

[John Wilander]

Non-Cocoa build fixes.

[youenn fablet]

Comment on attachment 353749 [details]
Patch

I think the patch is going in the right direction.
Some improvement comments and questions below.

View in context: https://bugs.webkit.org/attachment.cgi?id=353749&action=review

> Source/WebKit/NetworkProcess/NetworkResourceLoader.cpp:236
> +            m_cacheEntryForValidation = WTFMove(entry);

I am not sure we should directly reuse m_cacheEntryForValidation.
When m_cacheEntryForValidation is not empty, we expect to send a conditional request and receive a 304.

It might be clearer if we introduce m_cacheEntryForMaxAgeCapValidation.
If not null, we do not have to do conditional request, check for 304 but instead to check whether the response is the same as the cached entry response.

> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:347
>          case UseDecision::NoDueToPrevalentResourceAgeCap:

Do we still need UseDecision::NoDueToPrevalentResourceAgeCap?
It seems the loader will do the check and cache is just the place that stores this information.

> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:423
> +std::unique_ptr<Entry> Cache::storeRedirect(const WebCore::ResourceRequest& request, const WebCore::ResourceResponse& response, const WebCore::ResourceRequest& redirectRequest, PAL::SessionID sessionID, CheckForMaxAgeCap shouldCheckForMaxAgeCap)

Instead of sessionID and shouldCheckForMaxAgeCap, we might want to directly pass an optional<Second>.
That will allow decoupling NetworkCache from NetworkStorageSession.

It seems NetworkStorageSession is more a loader/data task concept.

> Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:476
> +std::unique_ptr<Entry> Cache::replaceRedirect(const WebCore::ResourceRequest& request, const Entry& existingEntry, const WebCore::ResourceResponse& response, const WebCore::ResourceRequest& redirectRequest, PAL::SessionID sessionID, CheckForMaxAgeCap shouldCheckForMaxAgeCap)

Ditto here.

> Source/WebKit/NetworkProcess/cache/NetworkCacheEntry.cpp:145
> +    // Optional max age cap.

Not sure we need this comment.

Should we also encode/decode this no matter whether RESOURCE_LOAD_STATISTICS is defined or not.
The reason is that, depending on whether RESOURCE_LOAD_STATISTICS is defined, we should bump or not the network cache version.
It might be simpler to just always encode it.

> Source/WebKit/NetworkProcess/cache/NetworkCacheStorage.cpp:504
> +        { }

Antti will know more but I would have thought we either would handle maxAgeCap as bodyHash or put it in the header or body data.

> LayoutTests/http/tests/resourceLoadStatistics/cap-cache-max-age-for-prevalent-resource.html:34
> +        // This should not trigger a successful cache hit and create another capped cache entry.

Do we have a test that ensures that the request doing the max age cap revalidation is not a conditional request?

[Antti Koivisto]

Comment on attachment 353749 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=353749&action=review

>> Source/WebKit/NetworkProcess/cache/NetworkCacheStorage.cpp:504
>> +        { }
> 
> Antti will know more but I would have thought we either would handle maxAgeCap as bodyHash or put it in the header or body data.

Right, the Storage layer is only concerned about storing bytes to the disk and includes no semantics. Like other similar things, maxAgeCap should be encoded as header data (in NetworkCache::Entry::encodeAsStorageRecord). Is there some good reason this patch moves it here?

[John Wilander]

(In reply to youenn fablet from comment #18)
> Comment on attachment 353749 [details]
> Patch
> 
> I think the patch is going in the right direction.
> Some improvement comments and questions below.
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=353749&action=review
> 
> > Source/WebKit/NetworkProcess/NetworkResourceLoader.cpp:236
> > +            m_cacheEntryForValidation = WTFMove(entry);
> 
> I am not sure we should directly reuse m_cacheEntryForValidation.

You are right. This is a leftover from when I tried to build this strictly on top of revalidation which turned out not to be a thing for redirects.

> When m_cacheEntryForValidation is not empty, we expect to send a conditional
> request and receive a 304.

Absolutely true.

> It might be clearer if we introduce m_cacheEntryForMaxAgeCapValidation.

Yes, I will do that.

> If not null, we do not have to do conditional request, check for 304 but
> instead to check whether the response is the same as the cached entry
> response.
> 
> > Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:347
> >          case UseDecision::NoDueToPrevalentResourceAgeCap:
> 
> Do we still need UseDecision::NoDueToPrevalentResourceAgeCap?
> It seems the loader will do the check and cache is just the place that
> stores this information.

Yeah, you might be right. I have to think it through.

> > Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:423
> > +std::unique_ptr<Entry> Cache::storeRedirect(const WebCore::ResourceRequest& request, const WebCore::ResourceResponse& response, const WebCore::ResourceRequest& redirectRequest, PAL::SessionID sessionID, CheckForMaxAgeCap shouldCheckForMaxAgeCap)
> 
> Instead of sessionID and shouldCheckForMaxAgeCap, we might want to directly
> pass an optional<Second>.
> That will allow decoupling NetworkCache from NetworkStorageSession.
> 
> It seems NetworkStorageSession is more a loader/data task concept.

Sounds like a good idea.

> > Source/WebKit/NetworkProcess/cache/NetworkCache.cpp:476
> > +std::unique_ptr<Entry> Cache::replaceRedirect(const WebCore::ResourceRequest& request, const Entry& existingEntry, const WebCore::ResourceResponse& response, const WebCore::ResourceRequest& redirectRequest, PAL::SessionID sessionID, CheckForMaxAgeCap shouldCheckForMaxAgeCap)
> 
> Ditto here.

Mhm.

> > Source/WebKit/NetworkProcess/cache/NetworkCacheEntry.cpp:145
> > +    // Optional max age cap.
> 
> Not sure we need this comment.
> 
> Should we also encode/decode this no matter whether RESOURCE_LOAD_STATISTICS
> is defined or not.
> The reason is that, depending on whether RESOURCE_LOAD_STATISTICS is
> defined, we should bump or not the network cache version.
> It might be simpler to just always encode it.

OK.

> > Source/WebKit/NetworkProcess/cache/NetworkCacheStorage.cpp:504
> > +        { }
> 
> Antti will know more but I would have thought we either would handle
> maxAgeCap as bodyHash or put it in the header or body data.
> 
> > LayoutTests/http/tests/resourceLoadStatistics/cap-cache-max-age-for-prevalent-resource.html:34
> > +        // This should not trigger a successful cache hit and create another capped cache entry.
> 
> Do we have a test that ensures that the request doing the max age cap
> revalidation is not a conditional request?

How would we test that? You mean if the server responds with a 304?

Thanks for all these comments!

[John Wilander]

(In reply to Antti Koivisto from comment #19)
> Comment on attachment 353749 [details]
> Patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=353749&action=review
> 
> >> Source/WebKit/NetworkProcess/cache/NetworkCacheStorage.cpp:504
> >> +        { }
> > 
> > Antti will know more but I would have thought we either would handle maxAgeCap as bodyHash or put it in the header or body data.
> 
> Right, the Storage layer is only concerned about storing bytes to the disk
> and includes no semantics. Like other similar things, maxAgeCap should be
> encoded as header data (in NetworkCache::Entry::encodeAsStorageRecord). Is
> there some good reason this patch moves it here?

Probably not. I will try to move it. Thanks!

[John Wilander]

Created attachment 353895 [details]
Patch

[John Wilander]

Created attachment 353896 [details]
Patch

[youenn fablet]

Comment on attachment 353896 [details]
Patch

LGTM with comments (I am not sure we should introduce replaceRedirect in particular).

We should probably bump the network cache revision number.
Since it was done one week ago, maybe this is not needed.
If we do not bump it, please add a comment in the ChangeLog for why we are not doing it.

As of the check of the conditional request, the server php script could check whether any of the request contains some If-Modified-Since/If-None-Match.. header. In such a case, it could generate a response that would make the test fail.

View in context: https://bugs.webkit.org/attachment.cgi?id=353896&action=review

> Source/WebKit/NetworkProcess/NetworkResourceLoader.cpp:601
> +#if ENABLE(RESOURCE_LOAD_STATISTICS)

Maybe we could put this new code in a new method called validateCacheEntryForMaxAgeCapValidation.
It would return a std::optional<Seconds>
If RESOURCE_LOAD_STATISTICS is not defined, it would return nullopt.
If m_cacheEntryForMaxAgeCapValidation is null, it would return networkStorageSession->maxAgeCacheCap(request) .
In case of replacing the entry, it would not use replaceRedirect, but would just call remove().
The readding of the redirect would happen later on, in the regular code path.

That would remove the need for replaceRedirect, which I am not sure is correct, since the new redirectResponse might not be cacheable.

Some comments below if you do not like the idea of the utility function/removal of replaceRedirect.

> Source/WebKit/NetworkProcess/NetworkResourceLoader.cpp:609
> +            m_cacheEntryForMaxAgeCapValidation = m_cache->replaceRedirect(request, *m_cacheEntryForMaxAgeCapValidation, redirectResponse, redirectRequest, std::nullopt);

No need to set m_cacheEntryForMaxAgeCapValidation since we nullify it shortly after without using it.
So maybe replaceRedirect should return void.

> Source/WebKit/NetworkProcess/NetworkResourceLoader.cpp:611
> +            m_cache->remove(m_cacheEntryForMaxAgeCapValidation->key());

If we remove the previous entry from the cache, do we still want to store the new one?
The patch does so which might be safer.

> Source/WebKit/NetworkProcess/NetworkResourceLoader.cpp:617
> +    if (!existingCacheEntryIsValid && redirectResponse.source() == ResourceResponse::Source::Network && canUseCachedRedirect(request)) {

I would rename existingCacheEntryIsValid to shouldTryCachingEntry.
That way we would write:
if (shouldTryCachingEntry && ...)
That seems to read better to me.

> Source/WebKit/NetworkProcess/cache/NetworkCacheSpeculativeLoad.cpp:82
> +#endif

I would reduce the code difference by doing:
std::optional<Seconds> maxAgeCap;
#if ENABLE(RESOURCE_LOAD_STATISTICS)
if (auto networkStorageSession = WebCore::NetworkStorageSession::storageSession(PAL::SessionID::defaultSessionID()))
    maxAgeCap = networkStorageSession->maxAgeCacheCap(request);
 #endif
m_cacheEntry = m_cache->storeRedirect(request, redirectResponse, redirectRequest, maxAgeCap);

[John Wilander]

Created attachment 354007 [details]
Patch for landing

[John Wilander]

Thanks Youenn and Antti! I broke out the functionality into NetworkResourceLoader::validateCacheEntryForMaxAgeCapValidation(), removed Cache::replaceRedirect(), and made sure the test case will fail if validation headers are sent.

[John Wilander]

Created attachment 354009 [details]
Patch for landing

[John Wilander]

The previous patch was missing the explainer in the change log. Now fixed.

[WebKit Commit Bot]

Comment on attachment 354009 [details]
Patch for landing

Clearing flags on attachment: 354009

Committed r237895: <https://trac.webkit.org/changeset/237895>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.