Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [PATCH 3/3] readahead: introduce context readahead algorithm | Date | Tue, 14 Apr 2009 23:43:32 -0400 |
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Wu Fengguang <fengguang.wu@intel.com> writes:
> Introduce page cache context based readahead algorithm. > This is to better support concurrent read streams in general. > > RATIONALE > --------- > The current readahead algorithm detects interleaved reads in a _passive_ way. > Given a sequence of interleaved streams 1,1001,2,1002,3,4,1003,5,1004,1005,6,... > By checking for (offset == prev_offset + 1), it will discover the sequentialness > between 3,4 and between 1004,1005, and start doing sequential readahead for the > individual streams since page 4 and page 1005. > > The context readahead algorithm guarantees to discover the sequentialness no > matter how the streams are interleaved. For the above example, it will start > sequential readahead since page 2 and 1002. > > The trick is to poke for page @offset-1 in the page cache when it has no other > clues on the sequentialness of request @offset: if the current requenst belongs > to a sequential stream, that stream must have accessed page @offset-1 recently, > and the page will still be cached now. So if page @offset-1 is there, we can > take request @offset as a sequential access. > > BENEFICIARIES > ------------- > - strictly interleaved reads i.e. 1,1001,2,1002,3,1003,... > the current readahead will take them as silly random reads; > the context readahead will take them as two sequential streams. > > - cooperative IO processes i.e. NFS and SCST > They create a thread pool, farming off (sequential) IO requests to different > threads which will be performing interleaved IO. > > It was not easy(or possible) to reliably tell from file->f_ra all those > cooperative processes working on the same sequential stream, since they will > have different file->f_ra instances. And NFSD's file->f_ra is particularly > unusable, since their file objects are dynamically created for each request. > The nfsd does have code trying to restore the f_ra bits, but not satisfactory.
Hi, Wu,
I tested out your patches. Below are some basic iozone numbers for a single NFS client reading a file. The iozone command line is:
iozone -s 2000000 -r 64 -f /mnt/test/testfile -i 1 -w
The file system is unmounted after each run to flush the cache. The numbers below reflect only a single run each. The file system was also unmounted on the NFS client after each run.
KEY --- vanilla: 2.6.30-rc1 readahead: 2.6.30-rc1 + your 10 readahead patches context readahead: 2.6.30-rc1 + your 10 readahead patches + the 3 context readahead patches. nfsd's: number of NFSD threads on the server
I'll note that the cfq in 2.6.30-rc1 is crippled, and that Jens has a patch posted that makes the numbers look at least a little better, but that's immaterial to this discussion, I think.
vanilla
nfsd's | 1 | 2 | 4 | 8 --------+---------------+-------+------ cfq | 43127 | 22354 | 20858 | 21179 deadline| 43732 | 68059 | 76659 | 83231
readahead
nfsd's | 1 | 2 | 4 | 8 --------+---------------+-------+------ cfq | 42471 | 21913 | 21252 | 20979 deadline| 42801 | 70158 | 82068 | 82406
context readahead
nfsd's | 1 | 2 | 4 | 8 --------+---------------+-------+------ cfq | 42827 | 21882 | 20678 | 21508 deadline| 43040 | 71173 | 82407 | 86583
Cheers, Jeff
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