RE: (2) [PATCH] dma-buf: system_heap: avoid reclaim for order 4

[Jaewon Kim]

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>--------- Original Message ---------
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>Sender : John Stultz <jstultz@xxxxxxxxxx>
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>Date : 2023-02-07 13:37 (GMT+9)
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>Title : Re: (2) [PATCH] dma-buf: system_heap: avoid reclaim for order 4
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> 
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>On Sat, Feb 4, 2023 at 7:02 AM Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote:
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>> Hello John Stultz, sorry for late reply.
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>> I had to manage other urgent things and this test also took some time to finish.
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>> Any I hope you to be happy with following my test results.
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>>
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>>
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>> 1. system heap modification
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>>
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>> To avoid effect of allocation from the pool, all the freed dma
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>> buffer were passed to buddy without keeping them in the pool.
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>> Some trace_printk and order counting logic were added.
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>>
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>> 2. the test tool
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>>
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>> To test the dma-buf system heap allocation speed, I prepared
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>> a userspace test program which requests a specified size to a heap.
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>> With the program, I tried to request 16 times of 10 MB size and
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>> added 1 sleep between each request. Each memory was not freed
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>> until the total 16 times total memory was allocated.
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>Oof. I really appreciate all your effort that I'm sure went in to
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>generate these numbers, but  this wasn't quite what I was asking for.
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>I know you've been focused on allocation performance under memory
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>pressure, but I was hoping to see the impact of __using__ order 0
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>pages over order 4 pages in real world conditions (for camera or video
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>recording or other use cases that use large allocations). These
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>results seem to be still just focused on the difference in allocation
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>performance between order 0 and order 4 with and without contention.
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>That said, re-reading my email, I probably could have been more clear
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>on this aspect.
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>> 3. the test device
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>>
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>> The test device has arm64 CPU cores and v5.15 based kernel.
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>> To get stable results, the CPU clock was fixed not to be changed
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>> in run time, and the test tool was set to some specific CPU cores
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>> running in the same CPU clock.
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>>
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>> 4. test results
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>>
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>> As we expected if order 4 exist in the buddy, the order 8, 4, 0
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>> allocation was 1 to 4 times faster than the order 8, 0, 0. But
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>> the order 8, 0, 0 also looks fast enough.
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>>
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>> Here's time diff, and number of each order.
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>>
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>> order 8, 4, 0 in the enough order 4 case
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>>
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>>          diff   8       4       0
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>>      665 usec   0       160     0
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>>    1,148 usec   0       160     0
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>>    1,089 usec   0       160     0
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>>    1,154 usec   0       160     0
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>>    1,264 usec   0       160     0
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>>    1,414 usec   0       160     0
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>>      873 usec   0       160     0
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>>    1,148 usec   0       160     0
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>>    1,158 usec   0       160     0
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>>    1,139 usec   0       160     0
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>>    1,169 usec   0       160     0
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>>    1,174 usec   0       160     0
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>>    1,210 usec   0       160     0
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>>      995 usec   0       160     0
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>>    1,151 usec   0       160     0
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>>      977 usec   0       160     0
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>>
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>> order 8, 0, 0 in the enough order 4 case
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>>
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>>          diff   8       4       0
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>>      441 usec   10      0       0
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>>      747 usec   10      0       0
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>>    2,330 usec   2       0       2048
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>>    2,469 usec   0       0       2560
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>>    2,518 usec   0       0       2560
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>>    1,176 usec   0       0       2560
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>>    1,487 usec   0       0       2560
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>>    1,402 usec   0       0       2560
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>>    1,449 usec   0       0       2560
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>>    1,330 usec   0       0       2560
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>>    1,089 usec   0       0       2560
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>>    1,481 usec   0       0       2560
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>>    1,326 usec   0       0       2560
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>>    3,057 usec   0       0       2560
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>>    2,758 usec   0       0       2560
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>>    3,271 usec   0       0       2560
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>>
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>> From the perspective of responsiveness, the deterministic
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>> memory allocation speed, I think, is quite important. So I
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>> tested other case where the free memory are not enough.
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>>
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>> On this test, I ran the 16 times allocation sets twice
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>> consecutively. Then it showed the first set order 8, 4, 0
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>> became very slow and varied, but the second set became
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>> faster because of the already created the high order.
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>>
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>> order 8, 4, 0 in low memory
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>>
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>>          diff   8       4       0
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>>      584 usec   0       160     0
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>>   28,428 usec   0       160     0
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>>  100,701 usec   0       160     0
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>>   76,645 usec   0       160     0
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>>   25,522 usec   0       160     0
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>>   38,798 usec   0       160     0
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>>   89,012 usec   0       160     0
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>>   23,015 usec   0       160     0
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>>   73,360 usec   0       160     0
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>>   76,953 usec   0       160     0
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>>   31,492 usec   0       160     0
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>>   75,889 usec   0       160     0
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>>   84,551 usec   0       160     0
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>>   84,352 usec   0       160     0
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>>   57,103 usec   0       160     0
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>>   93,452 usec   0       160     0
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>>
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>>          diff   8       4       0
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>>      808 usec   10      0       0
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>>      778 usec   4       96      0
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>>      829 usec   0       160     0
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>>      700 usec   0       160     0
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>>      937 usec   0       160     0
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>>      651 usec   0       160     0
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>>      636 usec   0       160     0
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>>      811 usec   0       160     0
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>>      622 usec   0       160     0
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>>      674 usec   0       160     0
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>>      677 usec   0       160     0
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>>      738 usec   0       160     0
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>>    1,130 usec   0       160     0
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>>      677 usec   0       160     0
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>>      553 usec   0       160     0
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>>    1,048 usec   0       160     0
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>>
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>>
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>> order 8, 0, 0 in low memory
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>>
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>>         diff    8       4       0
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>>   1,699 usec    2       0       2048
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>>   2,082 usec    0       0       2560
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>>     840 usec    0       0       2560
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>>     875 usec    0       0       2560
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>>     845 usec    0       0       2560
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>>   1,706 usec    0       0       2560
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>>     967 usec    0       0       2560
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>>   1,000 usec    0       0       2560
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>>   1,905 usec    0       0       2560
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>>   2,451 usec    0       0       2560
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>>   3,384 usec    0       0       2560
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>>   2,397 usec    0       0       2560
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>>   3,171 usec    0       0       2560
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>>   2,376 usec    0       0       2560
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>>   3,347 usec    0       0       2560
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>>   2,554 usec    0       0       2560
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>>
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>>        diff     8       4       0
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>>  1,409 usec     2       0       2048
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>>  1,438 usec     0       0       2560
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>>  1,035 usec     0       0       2560
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>>  1,108 usec     0       0       2560
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>>    825 usec     0       0       2560
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>>    927 usec     0       0       2560
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>>  1,931 usec     0       0       2560
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>>  2,024 usec     0       0       2560
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>>  1,884 usec     0       0       2560
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>>  1,769 usec     0       0       2560
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>>  2,136 usec     0       0       2560
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>>  1,738 usec     0       0       2560
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>>  1,328 usec     0       0       2560
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>>  1,438 usec     0       0       2560
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>>  1,972 usec     0       0       2560
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>>  2,963 usec     0       0       2560
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>
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>So, thank you for generating all of this. I think this all looks as
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>expected, showing the benefit of your change to allocation under
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>contention and showing the potential downside in the non-contention
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>case.
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>I still worry about the performance impact outside of allocation time
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>of using the smaller order pages (via map and unmap through iommu to
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>devices, etc), so it would still be nice to have some confidence this
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>won't introduce other regressions, but I do agree the worse case
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>impact is very bad.
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>> Finally if we change order 4 to use HIGH_ORDER_GFP,
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>> I expect that we could avoid the very slow cases.
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>>
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>Yeah. Again, this all aligns with the upside of your changes, which
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>I'm eager for.
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>I just want to have a strong sense of any regressions it might also cause.
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>I don't mean to discourage you, especially after all the effort here.
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>Do you think evaluating the before and after impact to buffer usage
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>(not just allocation) would be doable in the near term?
>

Hello sorry but I don't have expertise on iommu. Actually I'm also wondering
all IOMMU can use order 4 free pages, if they are allocated. I am not sure
but I remember I heard order 9 (2MB) could be used, but I don't know about order 8 4.

I guess IOMMU mmap also be same patern like we expect. I mean if order 4 is
prepared it could be faster like 1 to 4 times. But it, I think, should NOT be
that much slow even though the entire free memory is prepared as order 0 pages.

>
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>If you don't think so, given the benefit to allocation under pressure
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>is large (and I don't mean to give you hurdles to jump), I'm willing
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>to ack your change to get it merged, but if we later see performance
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>trouble, I'll be quick to advocate for reverting it.  Is that ok?
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Yes sure. I also want to know if it is.
Thank you

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>thanks
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>-john
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>