On Wed, 3 Sep 2008, Igor Sysoev wrote:
On Wed, Sep 03, 2008 at 10:44:46AM -0600, Scott Long wrote:
On Wed, 3 Sep 2008, Igor Sysoev wrote:
On Wed, Sep 03, 2008 at 03:39:55PM +0300, Kostik Belousov wrote:
On Wed, Sep 03, 2008 at 01:53:52PM +0400, Igor Sysoev wrote:
Hi,
could anyone tell what does vfs.ffs.rawreadahead enable ?
As I understand it's used in DIRECTIO code that allows read data
directly to an userland buffer bypassing the buffer cache.
What I can not understand where the read ahead data can be placed in ?
The operation of the ffs_rawread is more accurately described as
bypassing the page cache. It creates the physical buffer that maps
the user pages.
The readahead is performed only when the supplied user memory region
is bigger then blocksize. In this case, two reads are performed
simultaneously, with both buffers mapping consequent blocks from
user-supplied buffers. The read operation looks like footsteps.
Nice!
As I understand the size limit of one read operation is MAXPHYS, which is
equal to 128K due to LBA28 ATA limit. On SCSI, SATA, and LBA48 ATA this
limit
can be increased. Is it safe ?
The value of MAXPHYS is unrelated to capabilities or limitations of ATA.
It was chosen based on the needs to prevent an excessive amount of
parallel I/O from exhausting the kernel address space and system memory.
In fact, the concern was with SCSI, not with ATA.
MAXPHYS can be raised, especially on 64bit platforms, but doing so also
bloats the sizes of a few key data structures. I've been looking at a
solution for this, and I'd rather that people keep their MAXPHYS changes
confined to their local trees rather than changing FreeBSD unless they
also solve the associated side effects.
As I understand MAXPHYS affects at least on pager_map size: on modern
machines it's usually 256 * MAXPHYS = 32M, therefore increasing MAXPHYS
will increase the map too.
This is intended and desirable.
The 128K is probably good value and I do not suggest to increase it by
default, I just want to increase MAXPHYS to improve disk throughput
on some hosts where nginx serves large files (1G+) using DIRECTIO.
I've tested increases up to 1M, and they all are very beneficial not
only for silly sequential style benchmarks but also for clustered i/o.
256-512k is the sweet spot, but Windows has set the standard at 1M and
I'd like to have FreeBSD follow suit eventually.
BTW, is it possible to change MAXPHYS to a loader tunnable ?
No. Struct buf is sized based on MAXPHYS, and there's no convenient way
yet to dynamically size that at runtime.
Scott
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