Hello, On Friday, January 28, 2011 4:31 PM Alan Stern wrote: > On Fri, 28 Jan 2011, Marek Szyprowski wrote: > > > This commit add code that flushed page cache disk buffers every 128 read > > and write request. It has almost no impact on the performance of the > > emulate usb drive but saves system page cache from being filled with > > useless emulated disk data. > > > > Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> > > Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx> > > > > --- > > > > Hello, > > > > In the embedded system development we encoutered a really anoying > > behaviour of USB mass storage gadget. We use it to export SD card as USB > > disk. However when user copies a lot of data from or to the USB disk, > > the mass storage gadget consumes almost all pages from system page > > cache. This results in a significant degradation of system and io > > performance. A lot of valid and really often used disk cache data is > > wiped from memory and need to be constantly reloaded. > > > > Caching mass storage backing file is usually useless. Host system very > > rarely sends requests for the same blocks in the limited time window. > > Usually there is another caching layer on the host system anyway. > > > > Disabling page cache for the backing file is also not the solution, > > because mmc perform really bad on direct 512 byte reads and writes. > > > > I've did a quick hack to limit the amount of page cache that is being > > used by mass storage gadget. I've forced mass storage gadget to flush > > and invalidate page cache once every 128 read or write request. I know > > that the patch is ugly but it solves the issue. > > Can you post an equivalent patch for file_storage.c? The same issues > apply there. Right. I wonder if my approach is really correct. Flushing every 128th request seems to be really hacky. I would really like to find a better way to decide when to flush/invalidate the page cache. I will post a patch for file_storage.c on Monday, now I need to go out from the office. > > Do you have any suggestion how to correctly introduce a method of > > limiting page cache usage in the mass storage gadget? > > How do other storage or filesystem servers do it (e.g., NFS, CIFS)? I will investigate how this is handled there. Thanks for the comments. Best regards -- Marek Szyprowski Samsung Poland R&D Center -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
