Re: [RFC v3 0/4] vfs: Non-blockling buffered fs read (page cache only)

[Michael Kerrisk]

Hello Milosz,

On Wed, Sep 24, 2014 at 11:46 PM, Milosz Tanski <milosz@xxxxxxxxx> wrote:
> This patcheset introduces an ability to perform a non-blocking read from
> regular files in buffered IO mode. This works by only for those filesystems
> that have data in the page cache.
>
> It does this by introducing new syscalls new syscalls preadv2/pwritev2. These
> new syscalls behave like the network sendmsg, recvmsg syscalls that accept an
> extra flag argument (RWF_NONBLOCK).
>
> It's a very common patern today (samba, libuv, etc..) use a large threadpool to
> perform buffered IO operations. They submit the work form another thread
> that performs network IO and epoll or other threads that perform CPU work. This
> leads to increased latency for processing, esp. in the case of data that's
> already cached in the page cache.
>
> With the new interface the applications will now be able to fetch the data in
> their network / cpu bound thread(s) and only defer to a threadpool if it's not
> there. In our own application (VLDB) we've observed a decrease in latency for
> "fast" request by avoiding unnecessary queuing and having to swap out current
> tasks in IO bound work threads.

Since this is a change to the user-space API, could you CC future
versions of this patch set to linux-api@xxxxxxxxxxxxxxxx please, as
per Documentation/SubmitChecklist. See also
https://www.kernel.org/doc/man-pages/linux-api-ml.html.

Thanks,

Michael


> Version 3 highlights:
>  - Down to 2 syscalls from 4; can user fp or argument position.
>  - RWF_NONBLOCK value flag is not the same O_NONBLOCK, per Jeff.
>
> Version 2 highlights:
>  - Put the flags argument into kiocb (less noise), per. Al Viro
>  - O_DIRECT checking early in the process, per. Jeff Moyer
>  - Resolved duplicate (c&p) code in syscall code, per. Jeff
>  - Included perf data in thread cover letter, per. Jeff
>  - Created a new flag (not O_NONBLOCK) for readv2, perf Jeff
>
>
> Some perf data generated using fio comparing the posix aio engine to a version
> of the posix AIO engine that attempts to performs "fast" reads before
> submitting the operations to the queue. This workflow is on ext4 partition on
> raid0 (test / build-rig.) Simulating our database access patern workload using
> 16kb read accesses. Our database uses a home-spun posix aio like queue (samba
> does the same thing.)
>
> f1: ~73% rand read over mostly cached data (zipf med-size dataset)
> f2: ~18% rand read over mostly un-cached data (uniform large-dataset)
> f3: ~9% seq-read over large dataset
>
> before:
>
> f1:
>     bw (KB  /s): min=   11, max= 9088, per=0.56%, avg=969.54, stdev=827.99
>     lat (msec) : 50=0.01%, 100=1.06%, 250=5.88%, 500=4.08%, 750=12.48%
>     lat (msec) : 1000=17.27%, 2000=49.86%, >=2000=9.42%
> f2:
>     bw (KB  /s): min=    2, max= 1882, per=0.16%, avg=273.28, stdev=220.26
>     lat (msec) : 250=5.65%, 500=3.31%, 750=15.64%, 1000=24.59%, 2000=46.56%
>     lat (msec) : >=2000=4.33%
> f3:
>     bw (KB  /s): min=    0, max=265568, per=99.95%, avg=174575.10,
>                  stdev=34526.89
>     lat (usec) : 2=0.01%, 4=0.01%, 10=0.02%, 20=0.27%, 50=10.82%
>     lat (usec) : 100=50.34%, 250=5.05%, 500=7.12%, 750=6.60%, 1000=4.55%
>     lat (msec) : 2=8.73%, 4=3.49%, 10=1.83%, 20=0.89%, 50=0.22%
>     lat (msec) : 100=0.05%, 250=0.02%, 500=0.01%
> total:
>    READ: io=102365MB, aggrb=174669KB/s, minb=240KB/s, maxb=173599KB/s,
>          mint=600001msec, maxt=600113msec
>
> after (with fast read using preadv2 before submit):
>
> f1:
>     bw (KB  /s): min=    3, max=14897, per=1.28%, avg=2276.69, stdev=2930.39
>     lat (usec) : 2=70.63%, 4=0.01%
>     lat (msec) : 250=0.20%, 500=2.26%, 750=1.18%, 2000=0.22%, >=2000=25.53%
> f2:
>     bw (KB  /s): min=    2, max= 2362, per=0.14%, avg=249.83, stdev=222.00
>     lat (msec) : 250=6.35%, 500=1.78%, 750=9.29%, 1000=20.49%, 2000=52.18%
>     lat (msec) : >=2000=9.99%
> f3:
>     bw (KB  /s): min=    1, max=245448, per=100.00%, avg=177366.50,
>                  stdev=35995.60
>     lat (usec) : 2=64.04%, 4=0.01%, 10=0.01%, 20=0.06%, 50=0.43%
>     lat (usec) : 100=0.20%, 250=1.27%, 500=2.93%, 750=3.93%, 1000=7.35%
>     lat (msec) : 2=14.27%, 4=2.88%, 10=1.54%, 20=0.81%, 50=0.22%
>     lat (msec) : 100=0.05%, 250=0.02%
> total:
>    READ: io=103941MB, aggrb=177339KB/s, minb=213KB/s, maxb=176375KB/s,
>          mint=600020msec, maxt=600178msec
>
> Interpreting the results you can see total bandwidth stays the same but overall
> request latency is decreased in f1 (random, mostly cached) and f3 (sequential)
> workloads. There is a slight bump in latency for since it's random data that's
> unlikely to be cached but we're always trying "fast read".
>
> In our application we have starting keeping track of "fast read" hits/misses
> and for files / requests that have a lot hit ratio we don't do "fast reads"
> mostly getting rid of extra latency in the uncached cases.
>
> I've performed other benchmarks and I have no observed any perf regressions in
> any of the normal (old) code paths.
>
>
> I have co-developed these changes with Christoph Hellwig.
>
> Milosz Tanski (4):
>   vfs: Prepare for adding a new preadv/pwritev with user flags.
>   vfs: Define new syscalls preadv2,pwritev2
>   vfs: Export new vector IO syscalls (with flags) to userland
>   vfs: RWF_NONBLOCK flag for preadv2
>
>  arch/x86/syscalls/syscall_32.tbl  |   2 +
>  arch/x86/syscalls/syscall_64.tbl  |   2 +
>  drivers/target/target_core_file.c |   6 +-
>  fs/cifs/file.c                    |   6 ++
>  fs/nfsd/vfs.c                     |   4 +-
>  fs/ocfs2/file.c                   |   6 ++
>  fs/pipe.c                         |   3 +-
>  fs/read_write.c                   | 121 +++++++++++++++++++++++++++++---------
>  fs/splice.c                       |   2 +-
>  fs/xfs/xfs_file.c                 |   4 ++
>  include/linux/aio.h               |   2 +
>  include/linux/fs.h                |   7 ++-
>  include/linux/syscalls.h          |   6 ++
>  include/uapi/asm-generic/unistd.h |   6 +-
>  mm/filemap.c                      |  22 ++++++-
>  mm/shmem.c                        |   4 ++
>  16 files changed, 163 insertions(+), 40 deletions(-)
>
> --
> 2.1.0
>
> --
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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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