On 06 Sep 2010, at 11:52 AM, Niklas Edmundsson wrote:
Regarding the issue of the disk cache cramming the entire file
into memory/address space, an alternate solution could be that the
cache returns buckets pointing to the cached file, ie that the
cache consumed those pesky mmapped buckets. This way the cache
could cache the file rather quickly independent of the speed of
the client, so that the file caching is finished in a sane time so
others can benefit from it even though the cache-initiating
request is running at snail-speed...
Been keen to do this for a while, this would definitely solve the
RAM problem, but wouldn't solve the time problem. Copying 4GB of
data from a slow disk can easily take minutes, and when Blu-ray
images start becoming common, the problem would get worse.
Yup. The next step to solve that would be to be able to serve
requests from cache while they are being cached. I don't know the
RFC implications of doing this, but in real life it's really useful.
With this in place, it becomes easy for an implementation to decide to
do "in flight" caching, plugging into the generic mod_cache provider
interface.
For those who have forgotten, that's what we do in our large-file-
caching-patchset for mod_disk_cache (hidden as an attachment to https://issues.apache.org/bugzilla/show_bug.cgi?id=39380
but I should really get around to upload an up2date version that
applies cleanly to the current 2.2 release). Some of the solutions
there aren't really applicable to httpd proper (mostly workarounds
for missing infrastructure), but some ideas are rather sane (like
writing the header files in a single go with an iovec with null
terminated strings instead of crlf-stuff thad needs to be parsed).
Oh, and the design caters for a shared data cache (ftp and rsync
access uses the same cache), which isn't really a priority for
something in httpd proper.
Given that the make-cache-writes-atomic problem requires a change to
the data format, it may be useful to look at this now, before v2.4 is
baked, which will happen soon.
How much of a performance boost is the use-null-terminated-strings?
Regards,
Graham
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