predictors: Reduce the maximum concurrent prefetches.

predictors: Reduce the maximum concurrent prefetches.

In the resource_prefetch_predictor, the maximum number of concurrent
prefetches is set to 24. This is too high, as:
- It defeats the prioritization of resources in the database
- It was shown to slow down page loads in some case on slow connections
  by starving the page load resource fetches. (local testing using WPR
  and tools in tools/resource_prefetch_predictor)

BUG=631966, 655980
Committed: https://crrev.com/e5c15db9908512b44a0a47f6b24098d8daeffeb1
Cr-Commit-Position: refs/heads/master@{#439453}

[Benoit L, 2016-12-19 10:06:01]

lizeb@chromium.org changed reviewers:
+ alexilin@chromium.org

[Benoit L, 2016-12-19 10:06:06]

The CQ bit was checked by lizeb@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-12-19 10:06:21]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[alexilin, 2016-12-19 10:24:00]

Isn't it too small? Do we have a data about the number of critical resources for
a typical page?
Do you know how many roundtrips can we make before the prefetcher will be
disabled? I'm afraid that we will have time for only one roundtrip (maybe two)
before a main page arrives. Thus we can assume that we will prefetch no more
than 10 resources with your limit. Is it enough?

[Benoit L, 2016-12-19 10:38:53]

On 2016/12/19 10:24:00, alexilin wrote:
> Isn't it too small? Do we have a data about the number of critical resources
for
> a typical page?
> Do you know how many roundtrips can we make before the prefetcher will be
> disabled? I'm afraid that we will have time for only one roundtrip (maybe two)
> before a main page arrives. Thus we can assume that we will prefetch no more
> than 10 resources with your limit. Is it enough?

Thanks for the comments.
You're right, setting an absolute limit is not the ideal solution, the proper
one being dependent on the current network connection throughput. On the other
hand, 24 is clearly too high, even for fast connections (seems to be an issue on
WiFi as well for some sites).

The number of round-trips depends on the scenario, that is navigation hint or
actual navigation. In all cases, we don't want to starve the main resource load.

So, let's set this to 5 for now, and revisit later through an experiment. What
do you think?

[alexilin, 2016-12-19 12:03:57]

Anyway we don't want to hurt page load performance so the lower limit is a
better start. We can consider improvements for this limit later. LGTM

The vital thing is css resources and I hope that the number of stylesheets
blocking the renderer will fit this limit.

[commit-bot: I haz the power, 2016-12-19 12:22:41]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-12-19 12:22:42]

Dry run: This issue passed the CQ dry run.

[Benoit L, 2016-12-19 12:36:00]

The CQ bit was checked by lizeb@chromium.org

[commit-bot: I haz the power, 2016-12-19 12:36:15]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-19 12:39:28]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1482150960211380, "parent_rev":
"84e9143850058cae9141a1ff535c852017dc6e83", "commit_rev":
"9d2425ea87361ff3a7c089ba1fcb85242442d72f"}

[commit-bot: I haz the power, 2016-12-19 12:39:54]

Description was changed from

==========
predictors: Reduce the maximum concurrent prefetches.

In the resource_prefetch_predictor, the maximum number of concurrent
prefetches is set to 24. This is too high, as:
- It defeats the prioritization of resources in the database
- It was shown to slow down page loads in some case on slow connections
  by starving the page load resource fetches. (local testing using WPR
  and tools in tools/resource_prefetch_predictor)

BUG=631966,655980
==========

to

==========
predictors: Reduce the maximum concurrent prefetches.

In the resource_prefetch_predictor, the maximum number of concurrent
prefetches is set to 24. This is too high, as:
- It defeats the prioritization of resources in the database
- It was shown to slow down page loads in some case on slow connections
  by starving the page load resource fetches. (local testing using WPR
  and tools in tools/resource_prefetch_predictor)

BUG=631966,655980

Review-Url: https://codereview.chromium.org/2589713002
==========

[commit-bot: I haz the power, 2016-12-19 12:39:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-12-19 12:42:13]

Description was changed from

==========
predictors: Reduce the maximum concurrent prefetches.

In the resource_prefetch_predictor, the maximum number of concurrent
prefetches is set to 24. This is too high, as:
- It defeats the prioritization of resources in the database
- It was shown to slow down page loads in some case on slow connections
  by starving the page load resource fetches. (local testing using WPR
  and tools in tools/resource_prefetch_predictor)

BUG=631966,655980

Review-Url: https://codereview.chromium.org/2589713002
==========

to

==========
predictors: Reduce the maximum concurrent prefetches.

In the resource_prefetch_predictor, the maximum number of concurrent
prefetches is set to 24. This is too high, as:
- It defeats the prioritization of resources in the database
- It was shown to slow down page loads in some case on slow connections
  by starving the page load resource fetches. (local testing using WPR
  and tools in tools/resource_prefetch_predictor)

BUG=631966,655980

Committed: https://crrev.com/e5c15db9908512b44a0a47f6b24098d8daeffeb1
Cr-Commit-Position: refs/heads/master@{#439453}
==========

[commit-bot: I haz the power, 2016-12-19 12:42:14]

Patchset 1 (id:??) landed as
https://crrev.com/e5c15db9908512b44a0a47f6b24098d8daeffeb1
Cr-Commit-Position: refs/heads/master@{#439453}