Here is how cache page replacement is done in Linux:
https://linux-mm.org/PageReplacementDesign
On Tue, Feb 7, 2017 at 5:32 AM, Niklas Edmundsson <ni...@acc.umu.se> wrote:
> On Mon, 6 Feb 2017, Jacob Champion wrote:
>
> <snip>
>
> Considering the massive amount of caching that's built into the entire
>> HTTP ecosystem already, O_DIRECT *might* be an effective way to do that (in
>> which we give up filesystem optimizations and caching in return for a DMA
>> into userspace). I have a PoC about halfway done, but I need to split my
>> time this week between this and the FCGI stuff I've been neglecting.
>>
>
> As O_DIRECT bypasses the cache, all your IO will hit your storage device.
> I think this makes it useful only in exotic usecases.
>
> For our workload that's using mod_cache with a local disk cache we still
> want to use the remaining RAM as a disk read cache, it doesn't make sense
> to force disk I/O for files that simply could have been served from RAM. We
> typically see 90-95% of the requests served by mod_cache actually being
> served from the disk cache in RAM rather than hitting the local disk cache
> devices. I'm suspecting the picture is similar for most occurrances of
> static file serving, regardless of using mod_cache etc or not.
>
> O_DIRECT also bypasses any read-ahead logic, so you'll have to do nice and
> big IO etc to get good performance.
>
> We've played around with O_DIRECT to optimize the caching process in our
> large-file caching module (our backing store is nfs). However, since all
> our hosts are running Linux we had much better results with doing plain
> reads and utilizing posix_fadvise with POSIX_FADV_WILLNEED to trigger
> read-ahead and POSIX_FADV_DONTNEED to drop the original file from cache
> when read (as future requests will be served from local disk cache). We're
> doing 8MB fadvise chunks to get full streaming performance when caching
> large files.
>
> But that's way out of scope for this discussion, I think ;-)
>
> In conclusion, I wouldn't expect any benefits of using O_DIRECT in the
> common httpd usecases. That said, I would gladly be proven wrong :)
>
> /Nikke
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> Niklas Edmundsson, Admin @ {acc,hpc2n}.umu.se | ni...@acc.umu.se
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