Fajun Chen wrote:
On 11/16/07, Mark Lord <liml@xxxxxx> wrote:
Fajun Chen wrote:
Hi All,
I use sg/libata and ata pass through for read/writes. Linux 2.6.18-rc2
and libata version 2.00 are loaded on ARM XScale board. Under heavy
cpu load (e.g. when blocks per transfer/sector count is set to 1),
I've observed that the test application can suck cpu away for long
time (more than 20 seconds) and other processes including high
priority shell can not get the time slice to run. What's interesting
is that if the application is under heavy IO load (e.g. when blocks
per transfer/sector count is set to 256), the problem goes away. I
also tested with open source code sg_utils and got the same result, so
this is not a problem specific to my user-space application.
..
Post the relevant code here, and then we'll be able to better understand
and explain it to you.
For example, if the code is using ATA opcodes 0x20, 0x21, 0x24,
0x30, 0x31, 0x34, 0x29, 0x39, 0xc4 or 0xc5 (any of the R/W PIO ops),
then this behaviour does not surprise me in the least. Fully expected
and difficult to avoid.
This problem also happens with R/W DMA ops. Below are simplified code snippets:
// Open one sg device for read
if ((sg_fd = open(dev_name, O_RDWR))<0)
{
...
}
read_buffer = (U8 *)mmap(NULL, buf_sz, PROT_READ | PROT_WRITE,
MAP_SHARED, sg_fd, 0);
// Open the same sg device for write
if ((sg_fd_wr = open(dev_name, O_RDWR))<0)
{
...
}
write_buffer = (U8 *)mmap(NULL, buf_sz, PROT_READ | PROT_WRITE,
MAP_SHARED, sg_fd_wr, 0);
..
Mmmm.. what is the purpose of those two mmap'd areas ?
I think this is important and relevant here: what are they used for?
As coded above, these are memory mapped areas taht (1) overlap,
and (2) will be demand paged automatically to/from the disk
as they are accessed/modified. This *will* conflict with any SG_IO
operations happening at the same time on the same device.
????
sg_io_hdr_t io_hdr;
memset(&io_hdr, 0, sizeof(sg_io_hdr_t));
io_hdr.interface_id = 'S';
io_hdr.mx_sb_len = sizeof(sense_buffer);
io_hdr.sbp = sense_buffer;
io_hdr.dxfer_len = dxfer_len;
io_hdr.cmd_len = cmd_len;
io_hdr.cmdp = cmdp; // ATA pass through command block
io_hdr.timeout = cmd_tmo * 1000; // In millisecs
io_hdr.pack_id = id; // Read/write counter for now
io_hdr.iovec_count=0; // scatter gather elements, 0=not being used
if (direction == 1)
{
io_hdr.dxfer_direction = SG_DXFER_TO_DEV;
io_hdr.flags |= SG_FLAG_MMAP_IO;
status = ioctl(sg_fd_wr, SG_IO, &io_hdr);
}
else
{
io_hdr.dxfer_direction = SG_DXFER_FROM_DEV;
io_hdr.flags |= SG_FLAG_MMAP_IO;
status = ioctl(sg_fd, SG_IO, &io_hdr);
}
...
Mmaped IO is a moot point here since this problem is also observed
when using direct IO.
Thanks,
Fajun
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