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From "Konstantin Chuguev" <Chug...@Clickstream.com>
Subject Excessive chunking [was: mod_disk_cache and atimes]
Date Wed, 26 Mar 2008 16:23:11 GMT
Thanks for the clarification.

A small correction: I meant writev() calls instead of sendfile() when  
working with small-size buckets.

The filter I'm developing provisionally splits the supplied buckets  
into relatively small buckets during content parsing. It then removes  
some of them and inserts some other buckets. Before passing the  
resulting brigade further down the filter chain, it merges all buckets  
that have their data in contiguous memory regions back together. So I  
guess I'm doing my bit in preventing excessive chunking.

I've done some research on the source files of httpd-2.2.6. The CORE  
filter seems to do de-chunking in the case when 16 or more buckets are  
passed to it (actually, the brigade is split if it contains flush  
buckets and each split part is checked for 16 buckets) AND the total  
amount of bytes in the 16 buckets does not exceed 8000. The filter  
then buffers the buckets together. Very clever.

	KC


On 26 Mar 2008, at 15:22, Dirk-Willem van Gulik wrote:
>
> On Mar 26, 2008, at 4:15 PM, Konstantin Chuguev wrote:
>
>> Can you please clarify your mentioning the bucket-brigade  
>> footprint? Are they so slow they make memory-based cache no more  
>> efficient then disk-based one? Or the opposite: sendfile() works so  
>> well that serving content from memory is not any faster?
>>
> No - they are very fast (in an absolute sense) - and your approach  
> is almost certainly the right one.
>
> However all-in-all there is a lot of logic surrounding them; and if  
> you are trying to squeeze out the very last drop (e.g. the 1x1 gif  
> example) - you run into all sorts of artificial limits, specifically  
> on linux and 2x2 core machines; as the memory which needs to be  
> accessed is just a little more scattered than one would prefer and  
> all sort of competition around the IRQ handling in the kernel and so  
> on.
>
> Or in other words - in a pure static case where you are serving very  
> small files which rarely if ever change, have no variance to any  
> inbound headers, etc - things are not ideal.
>
> But that is a small price to pay - i.e. apache is more of a swiss  
> army knife; which saw's OK, but a proper hacksaw is 'better'.
>
>> I'm developing an Apache output filter for highly loaded servers  
>> and proxies that juggles small-size buckets and brigades  
>> extensively. I'm not at the stage yet where I can do performance  
>> tests but if I knew this would definitely impact performance, I  
>> would perhaps switch to fixed-size buffers straight away...
>
>
> I'd bet you are on the right track. However there is -one- small  
> concern; sometimes if you have looooots of buckets and very chunked  
> output - then one gets lots and lots of 1-5 byte chunks; each  
> prefixed by the length byte. And this can get really inefficient.
>
> Perhaps we need a de-bucketer to 'dechunk' when outputting chunked.
>
> Dw
>

Konstantin Chuguev
Software Developer

Mobile: +44 7734 955973
Fax: + 44 20 7509 9600
Clickstream Technologies PLC, 58 Davies Street, London, W1K 5JF,  
Registered in England No. 3774129



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