Re: [PATCH RFC] fat: use write_inode_now() on flush to avoid racing the flusher

[Bryanna Marihugh <bryannamarihugh@manhattantech.edu>]

OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> writes:

> > Bryanna Marihugh <bryannamarihugh@manhattantech.edu> writes:
> >
> >> There is some regression with small files on the uas drives from the
> >> wait.
> >
> > Is this wait=1? The flush should just be early flush, no guarantee to
> > flush completely. So I think it should be wait=0.

Everything I have posted so far is wait=1. I just wanted to provide the
benchmarks for my existing patch. I have a wait=0 one I have been
testing now. I labeled it in the benchmarks.

> > And the flush is assuming basically for removable media though, we can't
> > choose. I think we need to check the result of such HDD/SSD.

I did tests with this machine's dirty_background_ratio=5 (the kernel
default is 10), which results in a 3.4-4 GiB threshold, and with a
256 MB threshold (dirty_background_bytes=256M,
dirty_writeback_centisecs=100) to test what a more RAM constrained
device might do. I set FSINFO next_cluster before each HDD run so all
three kernels write to the same part of the disk.

WD Blue 500G 5400RPM HDD SATA
copying a 4 GiB file 3x, then sync, MB/s
(requests and average size are from the third run, which was traced)

  5% threshold
    stock:  89.4, 86.9, 87.8  (8,913 requests, avg 471 KB)
    wait=1: 97.9, 97.3, 95.2  (2,168 requests, avg 1,951 KB)
    wait=0: 96.2, 95.7, 98.7  (1,626 requests, avg 2,580 KB)

  256 MB threshold
    stock:  81.0, 88.1, 81.4  (16,291 requests, avg 258 KB)
    wait=1: 100.7, 99.7, 98.4 (2,257 requests, avg 1,859 KB)
    wait=0: 101.9, 100.5, 98.7 (1,532 requests, avg 2,738 KB)

WD Blue 500G, copying 1000 x 1 MiB files 2x, then sync, MB/s

  5% threshold
    stock:  9.44, 9.45
    wait=1: 9.34, 9.32
    wait=0: 9.55, 9.58


Samsung SSD 870 EVO SATA, copying a 4 GiB file 3x, then sync, MB/s
(requests and average size are from the third run, which was traced)

The wait=0 runs were on a different day.

  5% threshold
    stock:  338, 340, 341     (1,460 requests, avg 2,874 KB)
    wait=1: 333, 339, 339     (1,366 requests, avg 3,071 KB)
    wait=0: 313, 313, 313     (1,792 requests, avg 2,441 KB)

  256 MB threshold
    stock:  377, 378, 377     (11,476 requests, avg 366 KB)
    wait=1: 378, 378, 378     (1,675 requests, avg 2,505 KB)
    wait=0: 346, 344, 345     (2,278 requests, avg 1,938 KB)

870 EVO, copying 1000 x 1 MiB files 2x, then sync, MB/s

  5% threshold
    stock:  9.61, 9.61
    wait=1: 8.92, 8.92
    wait=0: 9.61, 9.61

PNY 32G USB 3.0 (usb-storage), copying a 3 GiB file 3x, then sync, MB/s
(requests and average size are from the third run, which was traced)

  5% threshold
    stock:  6.4, 6.9, 6.9     (12,481 requests, avg 252 KB)
    wait=1: 17.5, 17.9, 18.0  (3,720 requests, avg 846 KB)
    wait=0: 17.9, 13.7, 16.4  (3,809 requests, avg 827 KB)

  256 MB threshold
    stock:  5.8, 6.1, 6.0     (13,996 requests, avg 225 KB)
    wait=1: 17.9, 16.4, 14.7  (3,575 requests, avg 881 KB)
    wait=0: 18.0, 18.2, 17.9  (3,663 requests, avg 860 KB)


PNY 32G stick, copying 1000 x 1 MiB files 2x, then sync, MB/s

  5% threshold
    stock:  5.49, 5.62
    wait=1: 5.82, 5.62
    wait=0: 5.66, 5.65

Samsung 64G USB stick (usb-storage),
copying a 4 GiB file 3x, then sync, MB/s
(requests and average size are from the third run, which was traced)

  5% threshold
    stock:  33.0, 33.0, 33.1  (12,725 requests, avg 330 KB)
    wait=1: 33.2, 33.1, 33.2  (4,521 requests, avg 928 KB)
    wait=0: 33.1, 33.2, 33.2  (4,583 requests, avg 915 KB)

  256 MB threshold
    stock:  33.3, 33.3, 33.3  (15,757 requests, avg 266 KB)
    wait=1: 33.3, 33.5, 33.5  (4,534 requests, avg 925 KB)
    wait=0: 33.5, 33.6, 33.5  (4,645 requests, avg 903 KB)


Samsung 64G stick, copying 1000 x 1 MiB files 2x, then sync, MB/s

  5% threshold
    stock:  7.34, 7.34
    wait=1: 7.33, 7.34
    wait=0: 7.32, 7.35

I was a bit surprised by the improvement on the HDD. I will see if I
can find a high end HDD.

With the SSD the 870 EVO I did the wait=0 test on a different day.
Not sure why but it seemed to be slower on the wait=0 test, I am
blaming the run being on a different day. I think performance is
probably mostly flat with the fix.

> And maybe it needs to check unmount time (check how many dirty was
> missing to flush).

wait=0 does come with that downside. If the flusher is already writing
the file at close, wait=0 skips it and unmount has to write the rest.
At 5% the 3 GiB file on the PNY stays under the threshold, so the
flusher doesn't start until the 30 second expiry. That's why stock is
still slow there and wait=0 behaves like wait=1. The 4 GiB files go
over it.

PNY 32G stick, 3 GiB file, cp then umount right away,
average of 3 runs (5% threshold) and 5 runs (256 MB):

5% threshold   cp returns  dirty at close  umount   total
stock               485 s            3 MB   0.6 s   485 s
wait=1              184 s            3 MB   0.8 s   185 s
wait=0              180 s            3 MB   0.6 s   180 s

256 MB         cp returns  dirty at close  umount   total
stock               513 s            3 MB   0.6 s   514 s
wait=1              176 s            2 MB   0.8 s   176 s
wait=0              2.6 s        3,004 MB   172 s   174 s


WD Blue 500G 5400RPM HDD SATA, 4 GiB file, cp then umount right away,
average of 3 runs (5% threshold) and 5 runs (256 MB):

5% threshold   cp returns  dirty at close  umount   total
stock              47.4 s            3 MB   0.8 s  48.2 s
wait=1             43.5 s            3 MB   1.2 s  44.7 s
wait=0              2.2 s        4,064 MB  42.3 s  44.5 s

256 MB         cp returns  dirty at close  umount   total
stock              51.3 s            3 MB   0.8 s  52.1 s
wait=1             44.0 s            2 MB   1.3 s  45.2 s
wait=0              2.0 s        4,021 MB  43.2 s  45.2 s


870 EVO SSD (internal SATA), 4 GiB file, cp then umount right away,
average of 3 runs (5% threshold) and 5 runs (256 MB):

5% threshold   cp returns  dirty at close  in flight  umount   total
stock              11.9 s            2 MB     362 MB   1.1 s  13.0 s
wait=1             12.2 s            2 MB       0 MB   0.9 s  13.0 s
wait=0              1.5 s        4,006 MB      22 MB  12.3 s  13.8 s

256 MB         cp returns  dirty at close  in flight  umount   total
stock              11.9 s            3 MB     112 MB   1.0 s  12.9 s
wait=1             11.9 s            2 MB       0 MB   0.9 s  12.8 s
wait=0              1.7 s        3,350 MB     180 MB   9.7 s  11.4 s


I can type up an actual patch with wait=0 and removing the helper
that Matheus suggested.

Thanks,
Bryanna