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From "Chris Schneider (JIRA)" <j...@apache.org>
Subject [jira] Commented: (NUTCH-385) Server delay feature conflicts with maxThreadsPerHost
Date Wed, 11 Oct 2006 18:44:36 GMT
    [ http://issues.apache.org/jira/browse/NUTCH-385?page=comments#action_12441528 ] 
            
Chris Schneider commented on NUTCH-385:
---------------------------------------

This comment was actually made by Andrzej in response to an email containing the analysis
above that I sent him before creating this JIRA issue:

Let's start with defining what is the desired semantics of these two parameters together.
In my opinion it's the following:

* if only 1 thread per host is allowed, at any given moment at most one thread should be accessing
the host, and the interval between consecutive requests should be at least crawlDelay (whichever
way we determine this value - from config, from robots.txt or external sources such as partner
agreements).

* if two or more (for example N) threads per host are allowed, at any given moment at most
N threads should be accessing the host, and the interval between consecutive requests should
be at least crawlDelay - that is, the interval between when one of the threads finishes, and
another starts requesting.

I.e.: for threads.per.host=2 and crawlDelay=3 seconds, if we start 3 threads trying to access
the same host we should get something like this (time in [s] on the x axis, # - start request,
+ - request in progress, b - blocked in per-host limit, c - obeying crawlDelay):

===0         1         2
===01234567890123456789012345678
1: #+++cccbbccc#++++cccbb#++++++
2: #++++++++cccbcccbcc#+++cccccb
3: bbbbccc#+++++ccc#+++++ccc#+++

As you can see, at any given time we have at most 2 threads accessing the site, and the interval
between consecutive requests is at least 3 seconds. Especially interesting in the above graph
is the period between 17-18 seconds - thread 2 had to be delayed additional 2 seconds to satisfy
the crawl delay requirement, even though the threads.per.host requirement was satisfied.

[snip]

It's a question of priorities - in the model I drafted above the topmost priority is the observance
of crawlDelay, sometimes at the cost of the number of concurrent threads (see seconds 17-18).
In this model, the code should always put the delay in BLOCKED_ADDR_TO_TIME, in order to wait
at least crawlDelay after _any_ thread finishes. We could use an alternative model, where
crawlDelay is measured from the start of the request, and not from the end - see the graph
below:

===0         1         2         3
===01234567890123456789012345678901234567
1: #+++cccccbbb#++++cccc#++++++cc#+++++++
2: ccc#++++++++cccccc#+++ccccc#++++c#++++
3: cccccc#+++++ccc#+++++ccc#+++++ccccccbb

but it seems to me that it's more complicated, gives less requests/sec, and the interpretaion
of crawlDelay's meaning is stretched ...

[snip]

> Server delay feature conflicts with maxThreadsPerHost
> -----------------------------------------------------
>
>                 Key: NUTCH-385
>                 URL: http://issues.apache.org/jira/browse/NUTCH-385
>             Project: Nutch
>          Issue Type: Bug
>          Components: fetcher
>            Reporter: Chris Schneider
>
> For some time I've been puzzled by the interaction between two paramters that control
how often the fetcher can access a particular host:
> 1) The server delay, which comes back from the remote server during our processing of
the robots.txt file, and which can be limited by fetcher.max.crawl.delay.
> 2) The fetcher.threads.per.host value, particularly when this is greater than the default
of 1.
> According to my (limited) understanding of the code in HttpBase.java:
> Suppose that fetcher.threads.per.host is 2, and that (by chance) the fetcher ends up
keeping either 1 or 2 fetcher threads pointing at a particular host continuously. In other
words, it never tries to point 3 at the host, and it always points a second thread at the
host before the first thread finishes accessing it. Since HttpBase.unblockAddr never gets
called with (((Integer)THREADS_PER_HOST_COUNT.get(host)).intValue() == 1), it never puts System.currentTimeMillis()
+ crawlDelay into BLOCKED_ADDR_TO_TIME for the host. Thus, the server delay will never be
used at all. The fetcher will be continuously retrieving pages from the host, often with 2
fetchers accessing the host simultaneously.
> Suppose instead that the fetcher finally does allow the last thread to complete before
it gets around to pointing another thread at the target host. When the last fetcher thread
calls HttpBase.unblockAddr, it will now put System.currentTimeMillis() + crawlDelay into BLOCKED_ADDR_TO_TIME
for the host. This, in turn, will prevent any threads from accessing this host until the delay
is complete, even though zero threads are currently accessing the host.
> I see this behavior as inconsistent. More importantly, the current implementation certainly
doesn't seem to answer my original question about appropriate definitions for what appear
to be conflicting parameters. 
> In a nutshell, how could we possibly honor the server delay if we allow more than one
fetcher thread to simultaneously access the host?
> It would be one thing if whenever (fetcher.threads.per.host > 1), this trumped the
server delay, causing the latter to be ignored completely. That is certainly not the case
in the current implementation, as it will wait for server delay whenever the number of threads
accessing a given host drops to zero.

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