Recent concurrency improvements in the AHCI driver and CAM need testing

Naoya Sugioka naoya.sugioka at
Sun Apr 10 23:41:10 PDT 2011

correction :)
 io_acpi  => io_apic

On Sun, Apr 10, 2011 at 11:37 PM, Naoya Sugioka <naoya.sugioka at> wrote:
> Hello,
> This is happened before your recent update, but my laptop showing
> CMD=15; timeout
> on ahci0.1 when io_acpi is enabled.  This timeout prevents to complete
> bootstrap process.
> I just wonder this is happened because ahci.0.1 is associated to ATAPI
> (DVD-RW) drive without
> occupant.
> dmesg telles:
> ahci0.1: Found ATAPI "TSSTcorp DVD+/-RW TS-U633F D200" serial="R3476GSSA81272"
> ahci0.1: tags=0/32 satacap=0202 satafea=0068 NCQ=NO capacity=1.00MB
> ahci0.1: f85=0000 f86=0000 f87=4000 WC=notsupp RA=notsupp SEC=notsupp
> then start showing a timeout message.
> Let me know if you need further information, thank you.
> -Naoya
> On Sat, Apr 9, 2011 at 9:00 PM, Matthew Dillon
> <dillon at> wrote:
>>    I've pushed some serious changes to the AHCI SATA driver and CAM.
>>    One fixes issues where the tags were not being utilized to their fullest
>>    extent... well, really they weren't being utilized at all.  I'm not
>>    sure how I missed the problem before, but it is fixed now.
>>    The second ensures that read requests cannot saturate all available
>>    tags and cause writes to stall, and vise-versa, and also separates
>>    out the read and write BIO streams and treats them as separate entities,
>>    which means that reads can continue to be dispatched even if writes
>>    saturate the drive's cache and writes can continue to be dispatched
>>    even if concurrent read(s) would otherwise eat all available tags.
>>    The reason the read/write saturation fixes are important is because
>>    writes are usually completed instantly since they just go to the drive
>>    cache, so even if reads are saturated there's no reason not to push
>>    writes to the drive.  Plus when the HD's cache becomes saturated writes
>>    no longer complete instantly and would prevent reads from being
>>    dispatched if all the tags were used to hold the writes.
>>    --
>>    With these fixes I am getting much better numbers with concurrency
>>    tests:
>>    I now get around 37000 IOPS doing random 512-byte sector reads with
>>    a Crucial C300 SSD, verses ~8000 or so before the fix.
>>    And I now get around ~365 IOPS with the same test on a hard drive,
>>    verses ~150 IOPS before (remember these are random reads!).
>>    blogbench also appears to have much better write/read parallelism
>>    against the swapcache with the SSD/HD combo.  Memory caches blow
>>    out at around blog #1300 on my test boxes.
>>        With the changes blogbench write performance is maintained through
>>        blog #1600 or so, without the changes it drops off at #1300.
>>        With the changes the swapcache SSD is pushing ~1400 IOPS or so
>>        satisfying random read requests.  Without the changes the swapcache
>>        SSD is only pushing ~130 IOPS.
>>        With the changes blogbench is able to maintain a ~60000 article
>>        read rate at the end of the test.  Without the changes the
>>        read rate is more around ~10000 at the end of the test.  At this
>>        stage swapcache has cached a significant chunk of the data
>>        in the SSD so the I/O activity is mixed random SSD and HD reads.
>>    --
>>    Ok, so I feel a bit sheepish that I missed the fact that the AHCI
>>    driver wasn't utilizing its tags properly before.  The difference
>>    in performance is phenominal.  Maybe we will start winning some
>>    of those I/O benchmark tests now.
>>                                        -Matt
>>                                        Matthew Dillon
>>                                        <dillon at>

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