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    SubjectPREEMPT_RT vs I-PIPE: the numbers, part 2
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    We've finally been able to complete a second round of our tests.
    Unfortunately, this has taken much longer than we ever anticipated.
    Without going into too much detail, suffice it to say that we ran into
    problems with each of our intended test configurations. Fortunately,
    the results are that much more interesting and, we hope, more
    accurately illustrate the performance of the approaches being
    compared.

    The setup is exactly the same as the one previously described. So if
    you've missed our earlier posting, it'll probably be easier to go read
    that one first for background info:
    http://marc.theaimsgroup.com/?l=linux-kernel&m=111846495403131&w=2

    Of course we would have liked to post the complete packages and
    configs that were used instead. Given the time spent on getting the
    tests to work again with the new configs, we were unable to put
    together a decent package though. Without further promising any
    dates, we will make those available shortly.

    Also, note that the versions of the approaches we were testing went
    through significant changes between those versions we last used and
    the versions published in the current testset.

    For PREEMPT_RT, word from Ingo is that a lot of things were fixed
    since the version we were using in our last tests. Hence, we used
    a more recent version and relied on the .config provided to us
    by Ingo. And indeed the numbers seem to confirm Ingo's analysis.
    As we will see below, PREEMPT_RT behaves quite well.

    For Adeos ... hmm Adeos? what's that ... I mean I-pipe ... well
    that's the news about it isn't it. While we were working on
    benchmarking his stuff, Philippe refactored the core component
    of the Adeos patch in order to isolate the interrupt pipeline
    therein implemented. Philippe's recent posting of these patches
    should provide a better explanation than we can put here. But
    basically, what we used and measured in Adeos is very much the
    same thing we are doing with the I-pipe here. Though the version
    posted Friday by Philippe to the LKML is more recent than the
    one we used for our tests, it should be good enough for comparison.

    Another change since the last test run is that in this case,
    both setups are compared to the same kernel, 2.6.12-rc6. Needless
    to say that we are much happier with comparing two approaches to
    the same exact kernel version instead of trying to correlate
    results from two different kernels.

    In terms of changes to the tests, a few minor things have
    changed. Most importantly, the hd test now correctly contains a
    "bs=1m", and hence results in the creation of a 512MB file on
    disk. Unlike the wishes we expressed earlier, we haven't integrated
    additional tests to the ones we had already carried out. It was
    enough work already to get a repeat of the same tests that
    followed the recommendations of the proponents of each method.
    Those tests that were mentioned remain relevant nonetheless,
    especially hackbench and dbench.

    So here are the results, and an attempted analysis. As was said
    earlier, we don't believe any single test run will ever
    definitively rule in favor or against either approach. Only
    continued benchmarking will help steer discussions we hope in
    the right direction.


    Total system load:
    ------------------
    Like last time, total system load is measured by LMbench's total
    execution time under various system loads. Again, these are on
    an average of 3 runs each, so the previous caveats still apply:
    on such a few runs, the numbers should be read as a general
    tendency with more definitive numbers requiring many more repeats.

    LMbench running times:
    +--------------------+-------+-------+-------+-------+-------+
    | Kernel | plain | IRQ | ping | IRQ & | IRQ & |
    | | | test | flood | ping | hd |
    +====================+=======+=======+=======+=======+=======+
    | Vanilla-2.6.12-rc6 | 175 s | 176 s | 185 s | 187 s | 246 s |
    +====================+=======+=======+=======+=======+=======+
    | with RT-V0.7.48-25 | 182 s | 184 s | 201 s | 202 s | 278 s |
    +--------------------+-------+-------+-------+-------+-------+
    | % | 4.0 | 4.5 | 8.6 | 8.0 | 13.0 |
    +====================+=======+=======+=======+=======+=======+
    | with Ipipe-0.4 | 176 s | 179 s | 189 s | 190 s | 260 s |
    +--------------------+-------+-------+-------+-------+-------+
    | % | 0.5 | 1.7 | 2.2 | 1.6 | 5.7 |
    +--------------------+-------+-------+-------+-------+-------+
    Legend:
    plain = Nothing special
    IRQ test = on logger: triggering target every 1ms
    ping flood = on host: "sudo ping -f $TARGET_IP_ADDR"
    IRQ & ping = combination of the previous two
    IRQ & hd = IRQ test with the following being done on the target:
    "while [ true ]
    do dd if=/dev/zero of=/tmp/dummy count=512 bs=1m
    done"

    In general, it seems that rc6 provides similar results as rc2 and
    rc4 did in our earlier tests, so that's a good indication of the
    validity of those numbers. The only number that is significantly
    different is the IRQ & hd test, but that is easily explained by
    the fact that in our earlier test we did not have the "bs=1m" in
    the "dd" command. Hence, only a 512 bytes file was generated
    instead of the intended 512MB.
    For PREEMPT_RT the numbers are also similar to those we found last
    time, which again helps confirm the tendency. However, as expected,
    the HD test is significantly different, for the same reasons as
    above (namely "bs=1m"). Without entering the world of speculation
    too much, it should be fair to say that the overhead generally
    observed in the various PREEMPT_RT runs in comparison to the
    vanilla runs is likely due to the additions introduced by the
    patch to some of the kernel's critical mechanisms.

    For the Ipipe, the numbers are better than those observed for Adeos
    in the last run for the light loads, but not as good for the heavy
    loads. While the I-pipe's impact remains much lower than PREEMPT_RT,
    as was the case for Adeos in the last test run, it is rather
    difficult to compare both test results as the I-pipe is not exactly
    Adeos. One important modification is that Adeos domains each ran
    using a separate stack and Adeos switched to the appropriate stack
    when switching domains. The current I-pipe has had this functionality
    stripped from it and, for what we make of it, the interrupts are
    handled on whatever stack exists at that time. It appears that this
    had the advantage of reducing the size of the patch, but if the
    I-pipe can be compared to Adeos, then it appears that removing this
    functionality has reduced performance. It is also possible that the
    slowdown may be due to subtle bugs introduced by the refactoring.
    More testing would need to be carried out to better determine the
    cause.
    In general, it appears that I-pipe's impact on general system
    performance is lower than PREEMPT_RT's.


    Interrupt response time:
    ------------------------
    Like last time, interrupt response time is measured by the delay it
    takes for the target to respond to interrupts from the logger. Given
    that somewhere between 500,000 and 650,000 interrupts are generated
    by the logger for each test run, we believe that these results
    illustrate fairly well the behavior of the measured approaches.

    Time in micro-seconds:
    +--------------------+------------+------+-------+------+--------+
    | Kernel | sys load | Aver | Max | Min | StdDev |
    +====================+============+======+=======+======+========+
    | | None | 13.9 | 55.5 | 13.4 | 0.4 |
    | | Ping | 14.0 | 57.9 | 13.3 | 0.4 |
    | Vanilla-2.6.12-rc6 | lm. + ping | 14.3 | 171.6 | 13.4 | 1.0 |
    | | lmbench | 14.2 | 150.2 | 13.4 | 1.0 |
    | | lm. + hd | 14.7 | 191.7 | 13.3 | 4.0 |
    +--------------------+------------+------+-------+------+--------+
    | | None | 13.9 | 53.1 | 13.4 | 0.4 |
    | | Ping | 14.4 | 56.2 | 13.4 | 0.9 |
    | with RT-V0.7.48-25 | lm. + ping | 14.7 | 56.9 | 13.4 | 1.1 |
    | | lmbench | 14.3 | 57.0 | 13.4 | 0.7 |
    | | lm. + hd | 14.3 | 58.9 | 13.4 | 0.8 |
    +--------------------+------------+------+-------+------+--------+
    | | None | 13.9 | 53.3 | 13.5 | 0.8 |
    | | Ping | 14.2 | 57.2 | 13.6 | 0.9 |
    | with Ipipe-0.4 | lm.+ ping | 14.5 | 56.5 | 13.5 | 0.9 |
    | | lmbench | 14.3 | 55.6 | 13.4 | 0.9 |
    | | lm. + hd | 14.4 | 55.5 | 13.4 | 0.9 |
    +--------------------+------------+------+-------+------+--------+
    Legend:
    None = nothing special
    ping = on host: "sudo ping -f $TARGET_IP_ADDR"
    lm. + ping = previous test and "make rerun" in lmbench-2.0.4/src/ on
    target
    lmbench = "make rerun" in lmbench-2.0.4/src/ on target
    lm. + hd = previous test with the following being done on the
    target:
    "while [ true ]
    do dd if=/dev/zero of=/tmp/dummy count=512 bs=1m
    done"

    In last week's test run it may have seemed that there was a distinct
    pattern for each setup and that average response times were slightly
    different (give or take a microsecond here or there.) In the current
    test run it appears that the average response time in all
    configurations is identical, that the same can be said about the
    minimum response times, and that vanilla clearly has much larger
    maximum response times when compared to either real-time extension.
    For PREEMPT_RT clearly the results are much better than last time.
    Indeed it appears that, as Ingo predicted, a combination of the
    proper configuration options and most recent additions gives
    PREEMPT_RT important gains. In comparison to last week's results
    all measures are lower: average response time, maximum response time,
    minimum response time, and standard deviation. This is very good.
    But that's not all. PREEMPT_RT also comes down virtually neck-to-
    neck with the I-pipe (and the previous numbers from Adeos) in
    terms of maximum interrupt response time. Certainly those backing
    PREEMPT_RT, and others we hope, will find this quite positive.

    The I-pipe, for its part, has yielded overall identical results to
    Adeos' results from last week. In doing so, it confirms its claims
    of inheriting Adeos' most important feature: the ability to obtain
    deterministic interrupt response times.


    Overall analysis:
    -----------------
    The guiding principal in carrying out those tests has been to help
    us, and we hope others, understand the impact the proposed real-time
    additions have on Linux. As such, these numbers are far from being
    the entire story. They only provide additional hints to those
    studying Linux's progression towards real-time responsiveness.

    Clearly the approaches analyzed here take a different path to
    their enabling of real-time in Linux. And while both are being
    submitted by separate groups to the general Linux community, it
    is important, as was made clear in earlier threads, to highlight
    that these approaches can and have already been used together --
    some have outright labeled them orthogonal. It follows that a
    comparison between these approaches should not necessarily be used
    for trying to determine a "winner".

    Instead, we suggest that the above tests will likely be used to
    better help those needing real-time responsiveness to decide which
    approach is best for them. As can be expected, such choices are
    often guided by compromise. We would have liked to offer some
    general guidelines, but after thinking long and hard, we thought
    it'd be best if those were laid out through the _constructive_
    criticism of the community. Nonetheless, here are some general
    _comments_ to start things off.

    On the face of it, one would be tempted to conclude that if you
    are looking for a lightweight mechanism for obtaining deterministic
    interrupt response times, then the I-pipe seems to be a pretty safe
    bet. After all, its impact on general system performance seems less
    than that of PREEMPT_RT.

    However, the I-pipe alone doesn't replace the functionality offered
    by PREEMPT_RT, most importantly in regards to the rt scheduling of
    user-space processes.

    So at this point one would be tempted to conclude that if you're
    looking for a fully integrated rt solution for Linux, PREEMPT_RT
    would be the best candidate. After all it seems to provide as
    good interrupt response times as I-pipe while providing rt
    scheduling of user-space processes, albeit with a price on
    general system performance.

    But as was stated earlier in these rt threads -- then the argument
    was made for Adeos, but it still applies for the I-pipe --, the
    I-pipe is also an enabler for running an rt executive side-by-side
    with Linux. With Fusion, for example, user-space processes can be
    provided with rt scheduling and other services using the
    transparent migration of service requests between Linux and the
    RT-executive.

    Again, as we said earlier, we do not believe that the approaches
    are mutually exclusive. In fact, given that the I-pipe can be
    easily integrated with PREEMPT_RT, it should be fairly simple to
    obtain a single Linux tree that supports both for the PREEMPT_RT
    approach and the I-pipe/Fusion approach.

    IFF a choice must be made between both PREEMPT_RT and the I-pipe/
    Fusion, then further tests would need to be carried out, in
    particular with regards to scheduling latencies. Based on
    guidance provided by previous discussion, however, such tests may
    actually reveal that, as in this case, the actual choice depends on
    a features vs. performance compromise.

    Which brings us back to Paul's earlier summary: it appears that
    there is not a single approach to solve all real-time problems
    in Linux. Instead, it seems that a clever combination of many
    approaches is the best way forward. In that regard, we hope that
    the above tests will help guide this analysis in a fruitful
    direction.

    As usual, your feedback is appreciated.

    Best regards,

    Kristian Benoit
    Karim Yaghmour
    --
    Pushing Embedded and Real-Time Linux Systems Beyond the Limits
    http://www.opersys.com || 1-866-677-4546



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