On Mon, Apr 30, 2001 at 07:57:52PM -0400, Cliff Woolley wrote:
> By "fairly expensive", I presume you mean this little block, which is
> linear with the number of files cached:
>
<exactly what I had in mind>
>
> It's certainly no worse than that.
Yes, even a linear scan scares me. I'm not the performance expert Dean
is, but that seems "fairly" expensive to me to do on *every* request.
mod_file_cache should be fast as we can make it. But, you could make the
case that the linear-scan tradeoff is worth it. My gut feeling is a
reader/writer lock implementation might scale better. But, I'd like to
see the code to prove that...
> You can even make it constant time by assuming that none of the files are
> mmaped to start with. Just before you serve a request, check to see if
> the file is MMAPed. If it's not, but it is after the request, mmaped_size
> += b->length again. But that might require some kind of locking, which is
> (I'm guessing) what you were getting at. Yeah, it could be a bit hairy to
> get a precise answer. An estimate might be sufficient and easier, I don't
> know for sure.
Yup. It really depends on the scale of what you are expecting
mod_file_cache to handle. I'd suppose that the more files you have, the
*more* you would want to use mod_file_cache. I doubt that a site with a
hundred-odd pages would even think about caching it. They'd get such a low
amount of traffic that almost any implementation of HTTP would do fine.
High-volume sites (such as cnet.com, for example) are sensitive to such
"fairly expensive" things, but can possibly get a big performance win by
leveraging mod_file_cache (although if they use SSIs that might not matter
much - since mod_file_cache wouldn't be involved...).
Didn't Ian just submit a patch to skip evaluation of some environment
variables in mod_include that increased his numbers by 25%? - if so,
that's big...
> At any rate, even if we don't try to track how much address space we've
> used up, it would still be way, way better after fixing the leak than what
> we have now, which uses up address space something like:
>
> sum_foreach_file_cached(sizeof(file)*num_requests_where_file_read).
>
> <shrug>
I know *anything* is an improvement over what is there right now. I'm
just thinking of the extreme cases for the proposed solution. This is a
minor quibble - don't mind me. =) -- justin
