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* Problem with read blocking for a long time on /dev/scd1
@ 2002-12-21  0:49 Gregory Stark
  2002-12-21  1:28 ` Greg Stark
                   ` (2 more replies)
  0 siblings, 3 replies; 10+ messages in thread
From: Gregory Stark @ 2002-12-21  0:49 UTC (permalink / raw)
  To: linux-kernel


I'm having a problem with ogle that seems to be being caused by the scsi or
ide-scsi driver. The video playback freezes for a second or randomly,
sometimes every few seconds, sometimes not for several minutes. Every such
glitch is correlated perfectly with a read syscall reading on /dev/scd1
blocking for an inordinate amount of time.

Most read syscalls from ogle seem to take between 30us to 100ms depending on
the size of the read. In fact plotting the time taken reported by strace -T vs
the size of the read in gnuplot produces a nice obvious linear correlation.

However occasionally there are outlier samples where the read call blocks for
between 150ms up to 1s or more. I've seen it block for about 10s once.

I've tried this on a machine that was essentially in single-user mode. The
only processes running were X, ogle, an xterm, and not much else. I'll include
my normal lsmod output below but the problem still occurred when playing with
hardly any modules loaded.

There's an FAQ about ogle glitching like this when something tries to probe
the cd drive, but nothing like that is running. And the glitches aren't
periodic like that FAQ describes.

The problem is definitely that the kernel simply doesn't schedule the process
calling read for over a second. 

There are no dmesg messages or other indication of problems in the ide-scsi or scsi
drivers.

Where would I start to track down what is preventing the kernel from
scheduling this process? It's presumably some problem in the ide or ide-scsi
driver, but I'm not sure where to start.

[I'm having trouble sending messages to the list, my ISPs mail server didn't
seem to deliver it the first time. My apologies if it finally makes it through
and this is a duplicate]

--
greg


^ permalink raw reply	[flat|nested] 10+ messages in thread
* Re: More tests [Was: Problem with read blocking for a long time on /dev/scd1]
@ 2003-01-08 12:03 Andrey Borzenkov
  0 siblings, 0 replies; 10+ messages in thread
From: Andrey Borzenkov @ 2003-01-08 12:03 UTC (permalink / raw)
  To: linux-kernel; +Cc: Greg Stark

> I've done some more tests:
>
> The problem still occurs with straight ide drivers, no ide-scsi
>
> The problem still occurs with 2.4.20-ac2
>
> I removed extraneous llseek syscalls from libdvdread, it's now reading
> purely sequentially and still failing. I doubt an llseek to the current
> location even gets through to the driver but I figured I would remove
> another variable.
> 
> Question: Does the readahead parameter in hdparm affect accesses to the
> raw /dev/hdd device? Does it affect atapi cdrom access?

I have seen the same phenomenon under slightly different conditions.

I am running Mandrake with supermount. In short, supermount fakes mounting device and then mounts real device on first access. On every file operation it checks for media change and invalidates and remounts media in this case.

In some cases, the best example being rpm -Uvh * on CD, this command lasts ages. The reason is long pause during file closing, as much as 5 seconds (in my case), so it takes very long time to examine all files. The device is pure IDE DVD-R (Creative) running in UDMA2 on i815e. I put timing information in ide-cd.c and ide.c and it shows that pause happens between ide_do_drive_cmd called from cdrom_queue_packet_command (from cdrom_check_status) and next corresponding ide_do_request (unfortunately, the log output happened inside of it, when request is already fetched).

Some strange things to observe: 

- this delay happens only during file close, while there are much more checks for media changes (every operation on supermount fs does it), the delayed ones are probably just 10% at most

- there is always a READ request being processed between ide_do_drive_cmd and ide_do_request; the actual delay happens between ide_do_drive_cmd and this READ request; I am going to trace all requests to CD_ROM to see when READ is generated

- when I try the above during relatively high disk activity, this usually works normally (i.e. without delays); OTOH when system is mostly idle I get these delays. I have pure IDE system, HD is on one channel, DVD in question on the other. I thought it may be related to DMA (packet commands are run with DMA disabled) but disabling DMA did not change anything.

The problem is old, it existed at least in 2.4.18 that shipped in Mandrake. Currently Mandrake is on 2.4.20 with the same problem.

I will do some tests to pinpoint the place where delays happen; I have not looked at ps output, but if delay happens in ide_do_request, there should not be many possibilities.

Regards

-andrej



^ permalink raw reply	[flat|nested] 10+ messages in thread
end of thread, other threads:[~2003-01-08 11:55 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2002-12-21  0:49 Problem with read blocking for a long time on /dev/scd1 Gregory Stark
2002-12-21  1:28 ` Greg Stark
2002-12-21  6:14 ` Greg Stark
2002-12-22 16:13   ` More tests [Was: Problem with read blocking for a long time on /dev/scd1] Greg Stark
2002-12-22 20:13     ` Jakob Oestergaard
2002-12-23  8:58       ` Greg Stark
2002-12-27 15:36         ` Jakob Oestergaard
2003-01-08  4:03         ` Jakob Oestergaard
2002-12-23 18:02 ` Problem with read blocking for a long time on /dev/scd1 Krzysztof Halasa
  -- strict thread matches above, loose matches on Subject: below --
2003-01-08 12:03 More tests [Was: Problem with read blocking for a long time on /dev/scd1] Andrey Borzenkov

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