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SubjectRe: (2) [PATCH 1/4] zstd: pass pointer rathen than structure to functions
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> On Jun 6, 2019, at 7:10 AM, Vaneet Narang <v.narang@samsung.com> wrote:
>
> Hi Andrew / David,
>
>
>>> > > - ZSTD_parameters params = ZSTD_getParams(level, src_len, 0);
>>> > > + static ZSTD_parameters params;
>>> >
>>> > > +
>>> > > + params = ZSTD_getParams(level, src_len, 0);
>>> >
>>> > No thats' broken, the params can't be static as it depends on level and
>>> > src_len. What happens if there are several requests in parallel with
>>> > eg. different levels?
>
> There is no need to make static for btrfs. We can keep it as a stack variable.
> This patch set focussed on reducing stack usage of zstd compression when triggered
> through zram. ZRAM internally uses crypto and currently crpto uses fixed level and also
> not dependent upon source length.

Can we measure the performance of these patches on btrfs and/or zram? See the benchmarks
I ran on my original patch to btrfs for reference https://lore.kernel.org/patchwork/patch/802866/.

I don't expect a speed difference, but I think it is worth measuring.

> crypto/zstd.c:
> static ZSTD_parameters zstd_params(void)
> {
> return ZSTD_getParams(ZSTD_DEF_LEVEL, 0, 0);
> }
>
>
> Actually high stack usage problem with zstd compression patch gets exploited more incase of
> shrink path which gets triggered randomly from any call flow in case of low memory and adds overhead
> of more than 2000 byte of stack and results in stack overflow.
>
> Stack usage of alloc_page in case of low memory
>
> 72 HUF_compressWeights_wksp+0x140/0x200
> 64 HUF_writeCTable_wksp+0xdc/0x1c8
> 88 HUF_compress4X_repeat+0x214/0x450
> 208 ZSTD_compressBlock_internal+0x224/0x137c
> 136 ZSTD_compressContinue_internal+0x210/0x3b0
> 192 ZSTD_compressCCtx+0x6c/0x144
> 144 zstd_compress+0x40/0x58
> 32 crypto_compress+0x2c/0x34
> 32 zcomp_compress+0x3c/0x44
> 80 zram_bvec_rw+0x2f8/0xa7c
> 64 zram_rw_page+0x104/0x170
> 48 bdev_write_page+0x80/0xb4
> 112 __swap_writepage+0x160/0x29c
> 24 swap_writepage+0x3c/0x58
> 160 shrink_page_list+0x788/0xae0
> 128 shrink_inactive_list+0x210/0x4a8
> 184 shrink_zone+0x53c/0x7c0
> 160 try_to_free_pages+0x2fc/0x7cc
> 80 __alloc_pages_nodemask+0x534/0x91c
>
> Thanks & Regards,
> Vaneet Narang
>

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